Dispersion and separation of nanostructured carbon in organic solvents

ABSTRACT

The present invention relates to dispersions of nanostructured carbon in organic solvents containing alkyl amide compounds and/or diamide compounds. The invention also relates to methods of dispersing nanostructured carbon in organic solvents and methods of mobilizing nanostructured carbon. Also disclosed are methods of determining the purity of nanostructured carbon.

This application claims the priority benefit of U.S. Provisional PatentApplication Ser. No. 60/623,322, filed Oct. 29, 2004, which is herebyincorporated by reference in its entirety.

The subject matter of this application was made with support from theUnited States Government under NSF Grant No. ECS0233776, NASA Grant No.NAG3-2828, and NASA Grant No. NCC3-053. The United States Government hascertain rights.

FIELD OF THE INVENTION

The present invention relates to dispersions of nanostructured carbon inorganic solvents, methods of dispersing and mobilizing nanostructuredcarbon, and methods of determining purity of nanostructured carbon.

BACKGROUND OF THE INVENTION

The unique electrical, optical, and mechanical properties inherent tocarbon nanostructures, such as single wall carbon nanotubes (“SWNTs”),have garnered tremendous interest in basic science and applied research(Avouris et al., J. Phys. B 323:6 (2002); Dai, Surf. Sci. 500:218(2002); and Landi et al., Nano Lett. 2:1329 (2002)). The opportunity toexploit these properties depends on the successful characterization andmanipulation of desired materials. In some cases, the necessity toutilize solution phase techniques is hindered by the inability to formstable SWNT dispersions. Many groups have resorted to functionalizationstrategies (Peng et al., Am. Chem. Soc. 125:15174 (2003); Kahn et al.,Nano Lett. 2:1215 (2002)), including the use of polymers (Landi et al.,Nano Lett. 2:1329 (2002); O'Connell et al., Chem. Phys. Lett.342:265-271 (2001)), surfactants (O'Connell et al., Science 297:593(2002); Matarredona et al., J. Phys. Chem. 107:13357 (2003)), and aminesto assist in dispersing SWNTs (Chen et al., Science 282:95 (1998); Chenet al., J. Phys. Chem. B 105:2525 (2001); Chattopadhyay et al., J. Am.Chem. Soc. 125:3370 (2003)). However, these techniques may disrupt SWNTstructure, alter electronic properties, or be problematic for subsequentremoval (Ausman et al., J. Phys. Chem. B 104:8911 (2000)). Therefore,the dispersion of as-produced, high aspect ratio, raw, and purifiedSWNTs in a suitable solvent is necessary to enable more accuratesolution phase analyses.

The most promising attempts at forming stable SWNT dispersions have beenwith organic amide solvents such as N,N-dimethylformamide (“DMF”) andN-methylpyrrolidone (“NMP”) (Ausman et al., J. Phys. Chem. B 104:8911(2000); Krupke et al., J. Phys. Chem. B 107:5667 (2003)), and with1,2-dichlorobenzene (“DCB”) for both HiPco and laser-generated SWNTs(Bahr et al., Chem. Commun. 2:193 (2001)). Calculation of the extinctioncoefficient at 2.48 eV (500 nm) for as-produced HiPco SWNTs in DCB wasreported to be 28.6 mL·mg⁻¹·cm⁻¹ (Bahr et al., Chem. Commun. 2:193(2001)). This is higher than the recently reported value of 9.7mL·mg⁻¹·cm⁻¹ for arc-discharge functionalized SWNTs in CS₂ at the sameenergy (Zhou et al., J. Phys. Chem. B 107:13588 (2003)). These resultsimply that variations exist for the extinction properties of SWNTmaterials, potentially occurring from differences in diameterdistributions, purity, and/or solvent effects. Dispersion of SWNTs inorganic amide solvents has been attributed to the availability of a freeelectron pair and high solvatochromic parameter, π*, although thesecharacteristics are not sufficient, since they are also present indimethyl sulfoxide (“DMSO”) which is inefficient at dispersing SWNTs(Ausman et al., J. Phys. Chem. B 104:8911 (2000)).

A variety of experimental methods can be employed in the fabrication ofSWNTs (i.e. arc-discharge, chemical vapor deposition, and pulsed laservaporization). However, each technique produces SWNTs with differingdiameter, chirality distributions, and various amounts of synthesisby-products (Dai, Surf. Sci. 500:218 (2002)). In general, theby-products are the principal component of the as-produced materials orraw SWNT “soot.” By-products such as graphitic and amorphous carbonphases, metal catalysts, fullerenes, and carbonaceous coatings on theSWNTs may not only dominate the physical characteristics of the rawsoot, but they also pose significant challenges in any subsequentpurification (Chiang et al., J. Phys. Chem. B 105:1157 (2001); Chiang etal., J. Phys. Chem. B 105:8297 (2001); Dillon et al., Adv. Mater.11:1354 (1999); Dillon et al., Mater. Res. Soc. Symp. Proc. 633:A5.2.1(2001); Harutyunyan et al., J. Phys. Chem. B 106:8671 (2002); Moon etal., J. Phys. Chem. B 105:5677 (2001); Strong et al., Carbon 41:1477(2003)). Further development of SWNT-based applications is expected torequire material standardization, specifically with respect toelectronic type and degree of purity. Consequently, there is a need todevelop a method whereby the types, amount, and morphology ofSWNT-containing materials can be accurately and precisely quantified(Arepalli et al., Carbon 42:1783 (2004)).

The present invention is directed to overcoming these and otherdeficiencies in the art.

SUMMARY OF THE INVENTION

One aspect of the present invention relates to a dispersion ofnanostructured carbon in an organic solvent, where the organic solventcontains an alkyl amide compound having the structure:

where

R₁ and R₂ are independently selected from the group consisting of H,C₁-C₆ alkyl, and phenyl and

R₃ is selected from the group consisting of H, C₁-C₆ alkyl, phenyl, and

where

Z is N or CH;

R₄, R₅, R₆, and R₇ are independently selected from the group consistingof H, C₁-C₆ alkyl, and phenyl; and

n is an integer from 1 to 3,

provided that when R₁ and R₂ are methyl, R₃ is not H.

Another aspect of the present invention relates to a method ofdispersing nanostructured carbon in an organic solvent. This methodinvolves providing nanostructured carbon and contacting thenanostructured carbon with an organic solvent containing an alkyl amidecompound, where the alkyl amide compound has a structure as describedabove.

A further aspect of the present invention relates to a method ofmobilizing nanostructured carbon. This method involves providing adispersion of nanostructured carbon in an organic solvent, where theorganic solvent contains an alkyl amide compound having a structure asdescribed above. An electrical field is applied to the dispersion underconditions effective to mobilize the nanostructured carbon.

Yet another aspect of the present invention relates to a dispersion ofnanostructured carbon in an organic solvent, where the organic solventcontains a diamide compound having the structure:

where

R₁ and R₂ are independently selected from C₁-C₆ alkyl and phenyl.

Yet a further aspect of the present invention relates to a method ofdispersing nanostructured carbon in an organic solvent. This methodinvolves providing nanostructured carbon and contacting thenanostructured carbon with an organic solvent containing a diamidecompound, where the diamide compound has a structure as described above.

Still another aspect of the present invention relates to a method ofmobilizing nanostructured carbon. This method involves providing adispersion of nanostructured carbon in an organic solvent, where theorganic solvent contains a diamide compound having a structure asdescribed above. An electrical field is applied to the dispersion underconditions effective to mobilize the nanostructured carbon.

Still a further aspect of the present invention relates to a method ofdetermining purity of nanostructured carbon. This method involvesproviding a dispersion of nanostructured carbon in an organic solvent,where the organic solvent contains an alkyl amide compound having astructure as described above. The dispersion is subjected to asolution-phase analysis under conditions effective to determine thepurity of the nanostructured carbon.

Another aspect of the present invention relates to a method ofdetermining purity of nanostructured carbon. This method involvesproviding a dispersion of nanostructured carbon in an organic solvent,where the organic solvent contains a diamide compound having a structureas described above. The dispersion is subjected to a solution-phaseanalysis under conditions effective to determine the purity of thenanostructured carbon.

The present invention relates to dispersion compositions of as-producedraw and purified laser-generated nanostructured carbon, as well asmixtures thereof, with several alkyl amide and diamide solvents. Animprovement in the dispersion limit can be obtained in comparison withsolvents DMF and NMP. The dispersion compositions of the presentinvention provide the ability to achieve well-resolved opticalabsorption spectra for nanostructured carbon characterization, withcorresponding higher extinction coefficients for purified materials.Additionally, the present invention relates to applications such assolution phase chromatography, scattering studies, and organic reactionchemistry using the dispersion compositions of the present invention.The use of stable nanostructured carbon/organic solvent dispersions inoptical absorption spectroscopy allows for a quantitative solution-phaseanalysis. Selection of an appropriate organic solvent permitshomogeneous sampling without chemical functionalization and simplesample recovery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-b are scanning electron microscopy (“SEM”) images of lasergenerated raw SWNT soot (FIG. 1 a) and purified, >95% w/w SWNTs (FIG. 1b). The thermogravimetric analysis (“TGA”) overlay in FIG. 1 c shows thethermal decomposition profile of FIG. 1 a and FIG. 1 b in air at a ramprate of 5° C./min.

FIG. 2 a is a photograph of the prepared 6.25 μg/mL SWNT-solventdispersions used to generate the optical absorption data shown in FIG. 2b. The colorless sample vial on the left is pure N,N-dimethylacetamide(“DMA”), (1) is raw SWNTs in DMA, (2) is raw SWNTs inN,N-dimethylpropanamide (“DMP”), (3) is raw SWNTs in DMF, and (4) is rawSWNTs in DEA. The gray bands in FIG. 2 b depict the energy transitionrange for the current SWNT diameter distribution. The arrows designatethe absorption maxima at 1.27 and 1.77 eV, corresponding to the secondsemiconducting and first metallic electronic transitions, respectively.

FIGS. 3 a-b are Beer's Law plots for each prepared SWNT-solventdispersion at 1.27 eV (FIG. 3 a) and 1.77 eV (FIG. 3 b). The dispersionlimit is estimated from the smooth curve fit at the last data pointbefore the inflection, and is designated by the vertical dashed lines.The linear trendline (---) generated using the four lowestconcentrations for the DMF dispersion series is provided to illustrateits dispersion limit. The experimental error is within the limits of thedata points.

FIG. 4 is an optical absorption spectra for stable SWNT dispersions at aprepared concentration of 6.25 μg/mL for DMA (1) and DCB (2).

FIG. 5 a is an optical absorption spectra of purified SWNT-DMAdispersions for the series of concentrations used to generate the Beer'sLaw analysis. FIG. 5 b is an optical absorption spectra comparing therelative absorbance intensity of (1) purified SWNTs, (2) raw SWNT soot,and (3) nanostructured carbon in DMA. The spectra, offset for clarity,were all obtained from 3.13 μg/mL concentrations. The gray bands in FIG.5 a and FIG. 5 b depict the energy transition range for the current SWNTdiameter distribution.

FIG. 6 is an optical absorption spectra for (1) purified SWNTs sprayedon quartz from acetone solution and (2) purified SWNTs in DMA.Improvement in the peak resolution is distinctly observed for the DMAdispersion compared to the dry solid.

FIGS. 7 a-c are SEM images at a magnification of 50,000× for purifiedSWNTs before (FIG. 7 a) and after (FIG. 7 b) dispersion in DMA. FIG. 7 cis a high magnification image of FIG. 7 b at 200,000×.

FIG. 8 is a graph showing optical absorption data for a dispersion ofpurified SWNT in N,N,N′,N′-tetramethyl-malonamide (“TMMA”).

FIG. 9 is a graph showing a dispersion limit curve for purifiedSWNT-TMMA dispersions derived from the peak maxima values in the opticalabsorption data.

FIGS. 10 a-b are SEM images of deposited SWNTs onto Copper electrodesafter 30 minutes at a bias of 300 V/m from a 1 μg/mL SWNT-DMAdispersion.

FIG. 11 is an optical absorption spectra for a 1 μg/mL stock solution ofpurified SWNTs in DMA. The overlay consists of extracted samples from anelectrophoretic separation of the stock solution under an applied fieldof 30 minutes and 60 minutes. The change in peak ratio(A_(1.27 eV)/A_(1.77 eV)) from stock solution to the 1^(st) extractindicates a significant shift in the semiconducting/metallic ratio ofthe SWNTs in solution.

FIGS. 12 a-f are SEM images for raw SWNTs synthesized using the pulsedlaser vaporization technique (“L-SWNT”) soot (FIG. 12 a); rawarc-discharged SWNTs (“A-SWNT”) soot (FIG. 12 b); purified L-SWNTs (FIG.12 c); purified A-SWNTs (FIG. 12 d); nanostructured carbon (FIG. 12 e);and carbon soot (FIG. 12 f). The magnifications for FIGS. 12 a, 12 b, 12e, and 12 f are 25000×. The magnifications for FIGS. 12 c and 12 d are100000×.

FIG. 13 is a graph showing an overlay of the Raman spectra fornanostructured carbon, carbon soot, purified L-SWNTs, and purifiedA-SWNTs at an incident laser energy of 1.96 eV.

FIGS. 14 a-b are optical absorption spectra for constructed sample setsof 2.5 μg/mL DMA dispersions for purified L-SWNTs and nanostructuredcarbon (NC) (FIG. 14 a); and purified A-SWNTs and carbon soot (CS) (FIG.14 b). The data for each sample set depict the highly resolved peakswhich are due to the interband electronic transitions associated withthe Van Hove singularities in SWNTs.

FIGS. 15 a-b are optical absorption spectra for 2.5 μg/mL DMAdispersions of purified L-SWNTs (FIG. 15 a) and purified A-SWNTs (FIG.15 b). The dashed line depicts a linear extrapolation of the two minimacorresponding to the ^(S)E₂₂ peak. The insets show the resulting spectraafter linear subtraction of the ^(S)E₂₂ peak based on the extrapolatedline.

FIG. 16 is a graph showing purity assessment results on the constructedsample sets shown in FIGS. 15 a-b for a ^(S)E₂₂ linear subtraction withthe ratio of areal absorbance to the purified SWNT reference. The closeddata points represent the purity assessment results using the equationreferenced in the art while the open data points are the correctedvalues using the equation of the present invention. The straight linerepresents the expected purity values for the constructed fractions ofSWNTs represented in the carbonaceous mass fraction (^(C)w_(SWNTs)).

FIG. 17 is an optical absorption spectra of purified L-SWNTs sprayedonto a quartz slide from a 0.1 mg/mL acetone solution with thecorresponding nonlinear π-plasmon curve fit shown by the dashed line.The Lorentzian curve fit from the data between 4.0-5.0 eV shows a strongcorrelation to the peak region of the data, and the peak maxima denotingthe π-plasmon energy is calculated to be 4.47 eV.

FIG. 18 is an optical absorption spectra of 2.5 μg/mL DMA dispersions of0% L-SWNTs (NC) and 100% L-SWNTs with the corresponding nonlinearπ-plasmon curve fits shown by dashed lines. The peak maxima denoting theπ-plasmon energy is calculated to be 4.84 eV for the 100% L-SWNTs and5.26 eV for the 0% L-SWNTs (NC). The gray band depicts the data regionfrom which the nonlinear π-plasmon curve fit was generated.

FIG. 19 is an optical absorption spectra of 2.5 μg/mL DMA dispersions of0% A-SWNTs (CS) and 100% A-SWNTs with the corresponding nonlinearπ-plasmon curve fits shown by dashed lines. The peak maxima denoting theπ-plasmon energy is calculated to be 4.90 eV for the 100% A-SWNTs and5.29 eV for the 0% A-SWNTs (CS). The gray band depicts the data regionfrom which the nonlinear π-plasmon curve fit was generated.

FIGS. 20 a-b are optical absorption spectral overlays for nonlinearπ-plasmon subtractions of the constructed sample sets for L-SWNTs (FIG.20 a) and A-SWNTs (FIG. 20 b).

FIGS. 21 a-b are calibration curves for purity assessment from thenonlinear π-plasmon subtracted constructed sample sets for L-SWNTs (FIG.21 a) and A-SWNTs (FIG. 21 b). The data was selected from the peakenergy values in FIGS. 20 a-b associated with the maximum absorbance at^(S)E₂₂ and ^(M)E₁₁ for each constructed fraction.

FIGS. 22 a-b are optical absorption spectra of 2.5 μg/mL DMA dispersionsfor raw L-SWNTs (FIG. 22 a) and raw A-SWNTs (FIG. 22 b) (as purchasedfrom Carbon Solutions, Inc.). The corresponding nonlinear π-plasmon fitsfor the raw SWNTs are represented by the dashed lines. The spectra forpurified SWNTs from each synthetic type are also overlaid for reference.The insets compare the π-plasmon subtracted results for each raw SWNTsoot against the 0% and 100% SWNT samples for each synthetic type.

FIG. 23 is an optical absorption spectra from FIG. 14 a for theconstructed L-SWNT sample set where the peak maxima for the ^(S)E₂₂ and^(M)E₁₁ transitions are highlighted with the symbols. The dashed tieline which is drawn between the peak maxima indicates the relativechanges in slope of this line between peaks for each SWNT fraction.

FIGS. 24 a-b are graphs showing rapid purity assessment methods onL-SWNT and A-SWNT constructed sample sets using the peak sum of theabsorbance maxima at ^(S)E₂₂ and ^(M)E₁₁ (FIG. 24 a) and slope from thetie lines between absorbance maxima at ^(S)E₂₂ and ^(M)E₁₁ (FIG. 24 b).The linear curve fits indicate the empirical relationships for eachmethod and SWNT synthesis type. The data sets have been normalized to 1μg SWNTs/mL DMA.

FIGS. 25 a-b are graphs showing rapid purity assessment methods on theL-SWNT and A-SWNT constructed sample sets using the maximum absorbancevalue for the ^(S)E₂₂ peak normalized to 1 μg SWNTs/mL DMA (FIG. 25 a);and the ratio of absorbance values for the ^(S)E₂₂ and ^(M)E₁₁ peaks(FIG. 25 b). The linear curve fits indicate the empirical relationshipsfor each method and SWNT synthesis type.

DETAILED DESCRIPTION OF THE INVENTION

One aspect of the present invention relates to a dispersion ofnanostructured carbon in an organic solvent, where the organic solventcontains an alkyl amide compound having the structure:

where

R₁ and R₂ are independently selected from the group consisting of H,C₁-C₆ alkyl, and phenyl and

R₃ is selected from the group consisting of H, C₁-C₆ alkyl, phenyl, and

where

Z is N or CH;

R₄, R₅, R₆, and R₇ are independently selected from the group consistingof H, C₁-C₆ alkyl, and phenyl; and

n is an integer from 1 to 3,

provided that when R₁ and R₂ are methyl, R₃ is not H.

In a preferred embodiment, the alkyl amide compounds of the presentinvention have the above structure where R₁, R₂, and R₃ areindependently selected from C₁-C₆ alkyl and phenyl. Even morepreferably, the alkyl amide compound is one or more of the followingcompounds:

-   DMA (N,N-dimethylacetamide) having the structure:

-   DMP (N,N-dimethylpropanamide) having the structure:

-   or N,N-diethylacetamide (“DEA”) having the structure:

In an alternative embodiment, R₃ of the alkyl amide compound of thedispersion of the present invention is preferably

and R₁, R₂, R₄, and R₅ are independently selected from C₁-C₆ alkyl andphenyl. Even more preferably,

Z is CH and

n is 1.

A particularly preferred alkyl amide compound of this embodimentincludes, without limitation, N,N-dimethylacetoacetamide having thestructure:

In another embodiment, R₃ of the alkyl amide compound of the dispersionof the present invention is preferably

R₁, R₂, R₄, and R₅ are independently selected from C₁-C₆ alkyl andphenyl;

Z is N and

n is 1.

Particularly preferred alkyl amide compounds of this embodiment of thedispersion of the present invention include, without limitation,N,N,N′,N′-tetramethylmalonamide having the structure:

-   N,N′-dibutyl-N,N′-dimethylmalonamide having the structure:

-   N,N,N′,N′-tetra(isopropyl)malonamide having the structure:

-   N,N,N′,N′-tetrahexylmalonamide having the structure:

-   2-methyl-N,N,N′,N′-tetrahexylmalonamide having the structure:

-   N,N,N′,N′-tetrahexyl-2,2-dimethylmalonamide having the structure:

Dispersions of the present invention are preferably stable dispersionswhich, in a preferred embodiment, do not contain externalfunctionalization agents such as polymers, surfactants, and amines toassist in dispersing the nanostructured carbon in the organic solvent.The dispersions are, in a preferred embodiment, stable for about a dayor, more preferably, are stable for about two days to a week or, evenmore preferably, are stable for more than a week and up to severalweeks.

For purposes of the present invention, nanostructured carbon is rawnanostructured carbon soot, purified nanostructured carbon, or mixturesthereof. Nanostructured carbon exists in a variety of forms including,but not limited to, carbon nanotubes, nano-onions, nano-horms, andfullerenes. In a preferred embodiment, the nanostructured carbon is inthe form of carbon nanotubes, such as single wall carbon nanotubes(SWNTs), double wall carbon nanotubes, multi-wall carbon nanotubes, ormixtures thereof. Most preferable, the nanostructured carbon is in theform of SWNTs.

Nanostructured carbon can be obtained commercially or prepared by avariety of synthetic routes known and practiced by those of ordinaryskill in the art. The synthesis of nanostructured carbon can beaccomplished in a wide variety of methods that involve the catalyticdecomposition of a carbon containing gas or solid. Some of the mostcommon techniques for the synthesis of carbon nanotubes, fullerenes,nanohoms, etc. are chemical vapor deposition, arc-discharge, and laservaporization synthesis. The synthesis conditions (e.g., temperature,pressure, carrier gas, etc.), metal catalyst type (most commonly iron,nickel, cobalt, or yttrium), and carbon source (graphite or hydrocarbon)have all been shown to influence the properties of the resulting carbonmaterials.

Nanostructured carbon dispersed by an alkyl amide solvent may be in morethan one form. For example, it may be desirable to form a dispersion ofnanostructured carbon containing two or more forms of nanostructuredcarbon. In a preferred embodiment, SWNTs are combined with amorphousnanostructured carbon to form a dispersion of the present invention. Itmay be desirable to combine SWNTs made in a synthesis reactor with ametal catalyst with amorphous nanostructured carbon made in the samereactor without the metal catalyst. It may be also be desirable todisperse nanostructured carbon in more than one form in predeterminedmass ratios.

Another aspect of the present invention relates to a method ofdispersing nanostructured carbon in an organic solvent. This methodinvolves providing nanostructured carbon and contacting thenanostructured carbon with an organic solvent containing an alkyl amidecompound, where the alkyl amide compound has a structure as describedabove.

Nanostructured carbon dispersed by an organic solvent according to thepresent invention, can be rendered mobile in solution under specificconditions. Preferably, two electrodes are placed opposite each other ina container that also contains the nanostructured carbon dispersion. Anelectric field is then applied between the electrodes. Thenanostructured carbon will migrate in the solution to one or bothelectrodes after sufficient time according to solvent properties (i.e.,dielectric constant) and solution temperature.

Thus, a further aspect of the present invention relates to a method ofmobilizing nanostructured carbon. This method involves providing adispersion of nanostructured carbon in an organic solvent, where theorganic solvent contains an alkyl amide compound having a structure asdescribed above. An electrical field is applied to the dispersion underconditions effective to mobilize the nanostructured carbon.

In a preferred embodiment, the mobilized nanostructured carbon isdeposited onto a substrate. Preferably, the substrate is a metalelectrode or a doped semiconductor.

Yet another aspect of the present invention relates to a dispersion ofnanostructured carbon in an organic solvent, where the organic solventcontains a diamide compound having the structure:

where

R₁ and R₂ are independently selected from C₁-C₆ alkyl and phenyl.

In a preferred embodiment, R₁ and R₂ of the diamide compound are methyl.

Yet a further aspect of the present invention relates to a method ofdispersing nanostructured carbon in an organic solvent. This methodinvolves providing nanostructured carbon and contacting thenanostructured carbon with an organic solvent containing a diamidecompound, where the diamide compound has a structure as described above.

Still another aspect of the present invention relates to a method ofmobilizing nanostructured carbon. This method involves providing adispersion of nanostructured carbon in an organic solvent, where theorganic solvent contains a diamide compound having a structure asdescribed above. An electrical field is applied to the dispersion underconditions effective to mobilize the nanostructured carbon.

Still a further aspect of the present invention relates to a method ofdetermining purity of nanostructured carbon. This method involvesproviding a dispersion of nanostructured carbon in an organic solvent,where the organic solvent contains an alkyl amide compound having astructure as described above. The dispersion is subjected to asolution-phase analysis under conditions effective to determine thepurity of the nanostructured carbon.

Solution-phase analysis can be performed on constructed sample sets thatvary the mass fraction of a purified component with respect to arepresentative carbonaceous impurity. The spectroscopic data from thesesample sets allow for numerous mathematical approaches to be applied inreference to a known metric of comparison. The application of anonlinear regression model using a Lorentzian subtraction of theπ-plasmon, as well as multiple rapid assessment protocols using theinterband electronic transitions (e.g., absolute absorbance intensity,peak maxima ratio, tie line slope, and a Beer's law analysis derivedfrom calculated extinction coefficients) has been developed for SWNTsand is described in detail in the Examples below.

In a preferred embodiment, determining the purity of nanostructuredcarbon involves applying a nonlinear regression of the nanostructuredcarbon π-plasmon. Alternatively, determining the purity ofnanostructured carbon involves using experimental extinctioncoefficients of the nanostructured carbon materials.

Suitable solution phase analyses include, without limitation,spectroscopy, optical absorption, fluorescence, nuclear magnetic, EPR,Raman, mass spectrometry, and chromatography. In a preferred embodiment,the solution phase analysis for carrying out the methods of the presentinvention is optical absorption spectroscopy.

Determining the purity of nanostructured carbon may be particularlyuseful when more than one type of nanostructured carbon is contained ina sample. For example, it may be desirable to determine the purity ofSWNTs in a sample containing SWNTs mixed with amorphous nanostructuredcarbon. Such mixtures may be made in predetermined mass ratios asdescribed above.

Another aspect of the present invention relates to a method ofdetermining purity of nanostructured carbon. This method involvesproviding a dispersion of nanostructured carbon in an organic solvent,where the organic solvent contains a diamide compound having a structureas described above. The dispersion is subjected to a solution-phaseanalysis under conditions effective to determine the purity of thenanostructured carbon.

EXAMPLES Example 1 SWNT Synthesis, Characterization, and Purification

SWNTs were synthesized using the pulse laser vaporization technique,employing an Alexandrite laser (755 nm) (Gennett et al., Mater. Res.Soc. Symp. Proc. 633:A9.1.1-A9.1.11 (2001), which is hereby incorporatedby reference in its entirety). The laser pulse was rastered using GSILumonics mirrors over the surface of a graphite (1-2 μm) target dopedwith 2% w/w Ni (sub-μm) and 2% w/w Co (<2 μm), at an average powerdensity of 100 W/cm². The reaction furnace temperature was held at 1150°C., with a chamber pressure of 700 torr under 100 sccm flowing Ar_((g)).The raw SWNT soot was collected from the condensed region on the quartztube at the rear of the furnace. Synthesis of a representativenanostructured carbon component in the raw soot was performed by laservaporization at the described conditions for an undoped graphite target.The nanostructured carbon material was devoid of SWNTs detectable bySEM, Raman, or optical absorption spectroscopies.

Analysis of SWNTs was performed by SEM, Raman spectroscopy, and TGA. SEMwas conducted using a Hitachi S-900, with samples applied directly tothe brass stub using silver paint. The instrument operated at anaccelerating voltage of 2 kV and magnifications ranged from 5000× to250000×. Raman spectroscopy was performed at room temperature using aJY-Horiba Labram spectrophotometer with excitation energies of 1.96 and2.54 eV. These energies have been shown to probe the metallic andsemiconducting laser-generated SWNTs, respectively, over the range ofdiameters used (Kataura et al., Synth. Met. 103:2555 (1999), which ishereby incorporated by reference in its entirety). Sample spectra wereobtained from 50 to 2800 cm⁻¹ using an incident beam attenuation filterto eliminate localized heating and subsequent sample decomposition. TGAwas conducted using a TA Instruments 2950. Samples were placed in theplatinum pan balance in quantities of ˜1 mg and ramped at 5° C./min fromroom temperature up to 950° C. under air at a gas flow rate of 60 sccmand N_(2(g)) balance purge at a gas flow rate of 40 sccm.

Purification of SWNT soot was performed using a modification of apreviously reported procedure (Dillon et al., Adv. Mater. 11:1354-1358(1999), which is hereby incorporated by reference in its entirety).Approximately 50 mg of raw SWNT soot was brought to reflux at 125° C. in3M nitric acid for 16 hours, and then filtered over a 1 μm PTFE membranefilter with copious amounts of water. The filter paper was rinsedconsecutively with acetone, ethanol, 2.5 M NaOH, and H₂O until filtratebecame colorless after each step. The membrane filter was dried at 70°C. in vacuo to release the resulting SWNT paper from the filter paper.The SWNT paper was thermally oxidized in air at 550° C. for 1 minute ina Thermolyne 1300 furnace. Finally, a 6M Hydrochloric acid wash for 60minutes using magnetic stirring, with similar filtering steps andthermal oxidation at 550° C. for 20 minutes, completed the purification.SEM and TGA analyses were conducted during the purification process toassure that the quality of the purification was at least 95% w/w SWNTs.In some cases, a post-purification annealing step has been employed at1100° C. under flowing Ar_((g)), which is expected to remove structuraldefects and surface functionalization (Martinez et al., Carbon 41:2247(2003), which is hereby incorporated by reference in its entirety).

Example 2 SWNT-Solvent Dispersion Preparation

Stable dispersions of SWNTs in the evaluated solvents were achievedusing a three-step process. Initially, stock solutions of 0.100 mgSWNTs/mL solvent were prepared and ultrasonicated (38.5-40.5 kHz) for 30minutes at 40° C. Serial dilutions with pure solvent were made toachieve the desired concentrations of 12.5, 6.25, 3.13, 1.56, 0.781, and0.391 μg/mL. The ultrasonication step was performed prior totransferring each aliquot to the subsequent dilution. Next, eachconcentration was centrifuged at 5,000 rpm for 10 min to remove anypotentially nondispersed material. The supernatant was decanted andanalyzed using optical absorption spectroscopy.

Example 3 Optical Absorption Spectroscopy

UV-vis-near-IR spectra were obtained using a Perkin-Elmer Lambda 900spectrophotometer. Sample handling for dispersion solutions involved theuse of 1 cm quartz cuvettes, while dry SWNT samples were air sprayedfrom a 0.1 mg/mL acetone solution onto 1 in² quartz slides. Theinstrument scanned over a wavelength range of 300-1600 nm at a datainterval of 1 nm. In the near-IR range, the instrument scan speed was375 nm/minute, with an integration time of 0.16 seconds, a 1.0 nm slit,and gain set at 1. For the UV-vis region, the scan speed was 375nm/minute, with an integration time of 0.12 seconds, a 4.0 nm slit, andthe gain set at 1.

It is well established that the diameter and electronic type of SWNTsare responsible for the unique set of transitions present in opticalabsorption spectroscopy (Kataura et al., Synth. Met. 103:2555 (1999),which is hereby incorporated by reference in its entirety). Thecharacteristic position of the absorption peaks correspond to abruptchanges in the electronic density of states, or Van Hove singularities.The ith pair of discrete electronic transition energies corresponding tothese singularities is approximated by the following:^(S,M) E _(ii)=2na _(c-c)γ₀ /d _(SWNT)where n is an integer, having values of 1, 2, 4, 5, or 7 forsemiconducting (S) SWNTs and n=3 or 6 for metallic (M) SWNTs in thespectral range of interest (Lian et al., J. Phys. Chem. B 107:12082(2003), which is hereby incorporated by reference in its entirety),a_(c-c) is the carbon-carbon bond distance with a value of 0.142 nm, andd_(SWNT) is the SWNT diameter (Odom et al., Nature 391:62 (1998);Wildoer et al., Nature 391:59 (1998), which are hereby incorporated byreference in their entirety). The carbon-carbon overlap integral, γ₀,for SWNTs has been reported to range from 2.45 to 3.0 eV (Hagen et al.,Nano Lett. 3:383 (2003), which is hereby incorporated by reference inits entirety). The electronic transitions probed in this opticalabsorption analysis are ^(S)E₂, ^(M)E₁₁, ^(S)E₃₃, ^(S)E₄₄, and ^(M)E₂₂,due to the absorption windows of the alkyl amide solvents.

Initially, characterization of the raw and purified SWNT materials wasperformed by SEM, TGA, and Raman spectroscopy. Shown in FIG. 1 a is arepresentative SEM image of raw SWNT soot, with the distinguishingcharacteristics of amorphous carbon and metal catalyst impuritiessurrounding the SWNT bundles. FIG. 1 b shows an SEM image of the sampleafter the purification process, evident by the removal of the synthesisimpurities. The TGA thermogram (see FIG. 1 c) indicates two prominentdecomposition temperatures for the raw SWNT soot at 450 and 553° C.,with a metal oxide residue of 7.5% w/w. These decomposition temperaturevalues are determined from the peak maxima of the first derivativeplots. The TGA data for the purified SWNTs shows a single decompositiontemperature at 677° C. with a metal oxide residue of 3.7% w/w. On thebasis of these data and an analysis using optical absorptionspectroscopy (Itkis et al., Nano Lett. 3:309 (2003), which is herebyincorporated by reference in its entirety), it is estimated that the rawsoot is ˜30% w/w SWNTs and the purified SWNTs is of a purity that is atleast 95% w/w. Raman spectroscopy of the laser-generated SWNTs has shownthe presence of both metallic and semiconducting SWNTs, each within adiameter range of 1.2 to 1.4 nm, on the basis of peak assignments of theradial breathing mode with the previously established relationshipbetween the Raman shift and SWNT diameter (Dresselhaus et al., Carbon40:2043 (2002), which is hereby incorporated by reference in itsentirety).

The ability to produce SWNT dispersions was first evaluated using asingle batch of raw SWNT soot with the following alkyl amide solvents:NMP, DMF, DMA, DEA, and DMP. The stability of these dispersions wasassessed after ultrasonication, centrifugation, and decantation steps.Optical absorption spectra were obtained for each prepared dispersionconcentration to establish the dispersion limit using Beer's Law plots(Strong, Anal. Chem. 24:338 (1952), which is hereby incorporated byreference in its entirety). This enabled both qualitative analysis ofthe dispersion limit for each solvent based on the presence of sedimentafter centrifugation and quantitative analysis using the absorbancemeasurements for each prepared dispersion concentration. In contrast tothe previous report, NMP was inefficient at dispersing the raw SWNT sootafter these processing steps. Shown in FIG. 2 a is a picture of theSWNT-solvent dispersions used during analysis, at a preparedconcentration of 6.25 μg/mL. The corresponding optical absorptionoverlay for these SWNT-solvent dispersions is represented in FIG. 2 b.The variation in absorbance intensity can be attributed to eachsolvent's ability to disperse the raw SWNT soot. The largest absorbanceand most well-resolved spectrum were exhibited by the SWNT-DMAdispersion. The peaks corresponding to the electronic transitions forsemiconducting (^(S)E₂₂, ^(S)E₃₃, and ^(S)E₄₄) and metallic (^(M)E₁₁)SWNTs are highlighted by the gray bands (Lian et al., J. Phys. Chem. B107:12082 (2003), which is hereby incorporated by reference in itsentirety). For this diameter distribution, the peak maxima for ^(S)E₂₂and ^(M)E₁₁ occur at 1.27 and 1.77 eV, respectively. These selectedenergies were used to generate dispersion limit curves from eachSWNT-solvent dispersion series.

The Beer's Law plot in FIG. 3 a shows the absorbance measurements at1.27 eV for SWNT-solvent dispersions over the concentration rangeexamined. The dispersion limit can be estimated from the plot bygenerating a smooth curve fit (Kaleidagraph) and observing the point atwhich the data deviates from a linear trendline. The extent of the“knee” in the inflection point can be attributed to density variationbetween solvents, with DMF having the highest density (0.944 g/mL) andDEA having the lowest (0.925 g/mL). Therefore, since DMF has thesmallest inflection, a linear trendline was generated from the fourlowest concentrations to better illustrate its dispersion limit.Additionally, it is observed that the absorbance values continue toincrease after the inflection point, resulting from the non-SWNTcarbonaceous materials present in raw soot that is dispersed in thesesolvents. The results for the estimated dispersion limits of raw SWNTsoot in each solvent are summarized in Table 1, but the general trend isas follows: DMA>DMP>DEA>DMF. The same trend was observed for absorbancemeasurements at 1.77 eV (FIG. 3 b), indicating that the dispersions arenot type sensitive, but include both metallic and semiconducting SWNTs.The dispersion limit values are listed as the last data point which wasconsistent with a linear trendline at an R²=0.999. In fact, thedispersion limit for DMA was a factor of four higher than that for DMF,(i.e. 6.25 μg/mL compared to 1.56 μg/mL), an improvement over previousreports using DMF (Ausman et al., J. Phys. Chem. B 104:8911 (2000);Krupke et al., J. Phys. Chem. B 107:5667 (2003), which are herebyincorporated by reference in their entirety). The dispersion limitcurves from the Beer's Law plot were corroborated by the presence ofsediment after centrifugation for the concentrations which deviated fromthe linear trendline. SEM analysis of the sediment showed a morphologywhich was indistinguishable from the raw SWNT soot, evident by thepresence of SWNTs, amorphous carbon, and metal catalyst impurities. Thestability of the SWNT-alkyl amide solvent dispersions was observed torange from 12-24 hours for concentrations near the dispersion limit, buton the order of days to a week for concentrations in the linear regionof the Beer's Law plot. The highest stability was observed for DMA, inagreement with the dispersion limit trend.

TABLE 1 Experimental Results for Dispersion Limit (D_(L)) Estimationsand Extinction Coefficients of Raw SWNTs in Alkyl Amide Solvents,Including the Reported Dielectric Constants ε_(1.27 eV) (raw ε_(1.77 eV)(raw D_(L) (raw SWNTs) SWNTs) SWNTs) (mL · mg⁻¹ · (mL · mg⁻¹ ·dielectric Solvent (μg/mL) cm⁻¹) cm⁻¹ constant DMF 1.56 23.4 30.0 37.06DMA 6.25 23.8 31.2 38.30 DBA 3.13 24.2 31.3 31.33 DMP 3.13 26.1 34.733.08

Previous work using DCB gives a higher dispersion limit of 95 μg/mL forraw HiPco, using a filtration method for estimating the dispersion limit(Bahr et al., Chem. Commun. 2:193 (2001), which is hereby incorporatedby reference in its entirety). A comparison on the use of DCB and DMA todisperse raw SWNT soot has been performed. The same SWNT concentrationseries used above was prepared for DCB and DMA through ultrasonicationat 40° C. No visual aggregates were observed in either solvent'ssolution for the 6.25 μg/mL prepared concentration. However, thepresence of non-dispersed particles was observed in both solvents forthe 12.5 μg/mL prepared concentrations. Unfortunately, due to the higherdensity of DCB (1.306 g/mL) compared to DMA (0.937 g/mL), thecentrifugation technique was unsuccessful at removing the non-dispersedparticles in DCB. In addition, filtration in the presence of glass woolwas seen to introduce variability in the absorption spectra of theresulting dispersions, possibly arising from the glass wool packingdensity and SWNT interactions during elution. Shown in FIG. 4 is anoverlay of the optical absorption spectra for the stable DCB and DMAdispersions at 6.25 μg/mL. The differences in absorbance values can beattributed to solvent effects, most notably the index of refraction forthe individual solvents (η²⁰ _(DCB)=1.551 and η²⁰ _(DMA)=1.438) (Skooget al., Principles of Instrumental Analysis, 5^(th) ed., Harcourt Brace& Co., Philadephia, Pa. (1998), which is hereby incorporated byreference in its entirety). Also, a blue shift of 22 meV was observedfor the absorption maximum in the DMA dispersion compared to the DCBdispersion, indicating a slight solvatochromic shift based on adifference in the electronic interactions between SWNTs and these twosolvents. DCB and DMA show similar dispersion limits (D_(L)) of6.25≦D_(L)<12.5 μg/mL for the raw SWNT soot, as compared to thepreviously reported value of 95 μg/mL. This may be attributed todifferences in the SWNT materials used, as well as the limited data setin the previous report, which may artificially inflate the dispersionlimit values due to a contribution from non-SWNT carbon (Bahr et al.,Chem. Commun. 2:193 (2001), which is hereby incorporated by reference inits entirety). Differences in the diameter distribution averages (thatfor HiPco is 1.0 nm, which is less than that for the laser, 1.37 nm) arewell known (Chattopadhyay et al., J. Am. Chem. Soc. 125:3370 (2003),which is hereby incorporated by reference in its entirety), but physicalproperties such as bundling, defect density, and purity are moredifficult to establish. The fact that both DCB and DMA show the abilityto form stable SWNT dispersions indicates that a set of structural andelectronic properties exists for these solvents, which are capable ofinteracting favorably with the SWNTs. A recent report has shown thatsonochemical polymerization of DCB enhances the dispersing ability ofthis solvent, but that this effect is not present with DMF solutions(Niyogi et al., J. Phys. Chem. B 107:8799 (2003), which is herebyincorporated by reference in its entirety). Similarly, DMA was evaluatedbefore and after ultrasonication and there was no observed change in theinfrared spectrum even after an exposure time of 4 hours. Therefore, thesonochemical polymerizations previously observed with DCB are notsuggested to be responsible for the stabilization of SWNTs in alkylamide solvents.

Evaluation of the Beer's Law plots for the absorption maxima of the rawSWNT-alkyl amide solvent dispersions has enabled the determination ofextinction coefficients. The values listed in Table 1, display a generaltrend for both semiconducting (1.27 eV) and metallic (1.77 eV)transitions, i.e. DMP>DEA≈DMA>DMF. This trend parallels the electrondonating character of the alkyl groups attached to the carbonyl of theamide. In addition, when the results for DEA and DMA are compared, thealkyl group present on the nitrogen shows a smaller, but consistent,increase in the extinction coefficient. As is common with alkenes, thesubstituents on the solvents will thermodynamically stabilize the doublebond character in the resonance stabilized amide, which leads tostabilization of the electronic dipole moment in the solvents, and inturn influences the interaction with an SWNT. This explanation issupported by the data, which shows a 10-15% increase in the extinctioncoefficient for DMP compared to DMF. Such a change is significant, sincethe difference in the index of refraction for DMP (η²⁰ _(DMP)=1.440) andDMF (η²⁰ _(DMF)=1.431) would not account for the absorbance increases.The apparent solvent interaction appears to increase the probability ofelectronic transitions in the SWNTs, resulting in the larger extinctioncoefficients, although no apparent solvatochromic shift is observedamong the four alkyl amide solvents for dispersions of raw soot. Theimportance of electron donor character toward stabilizing SWNTs insolution has been cited for DMF (Ausman et al., J. Phys. Chem. B104:8911 (2000), which is hereby incorporated by reference in itsentirety). However, the current alkyl amide solvents (DMA, DEA, and DMP)have even greater electron donating ability, leading to higherdispersion capability than observed in DMF. The polarity of thesesolvents, evident by the dielectric constants (see Table 1), may be animportant consideration, since the resulting electronic interaction islargely influenced by changes in the Van der Waals forces of the SWNTswith the solvent molecules. However, similar attempts at dispersingSWNTs using acetonitrile (dielectric constant 36.00) and DMSO(dielectric constant 46.71) (Laurence et al., J. Phys. Chem. B 98:5807(1994), which is hereby incorporated by reference in its entirety) havebeen unsuccessful, indicating that dispersion capability is not merely afunction of polarity. In addition to the dipole-dipole effects, theinteraction between alkyl amide solvents and SWNTs may result fromincreased π orbital overlap or “stacking,” (Chen et al., J. Am. Chem.Soc. 123:3838 (2001), which is hereby incorporated by reference in itsentirety) which is greatest for optimized solvent geometries. Forexample, the π orbital interaction would be significantly higher for DCBcompared to alkyl amide solvents, but the reduced polarity (dielectricconstant 10.36) (Laurence et al., J. Phys. Chem.B 98:5807 (1994), whichis hereby incorporated by reference in its entirety) may account for theequivalent dispersion performance. Therefore, in the case of alkyl amidesolvents, the presence of a highly polar π system in conjunction withappropriate bond lengths and bond angles should show the highest SWNTdispersion limit.

To maximize the electronic effects between the alkyl amide solvents andSWNTs, a complementary set of steric properties may be necessary. AM1theoretical calculations were performed using Chem 3D MOPAC, ageneral-purpose semiempirical quantum mechanics package, to determinethe geometries associated with the alkyl amide solvents (Arora, Asian J.Chem. 14:1719 (2002), which is hereby incorporated by reference in itsentirety). Table 2 lists the calculated bond lengths and bond anglesassociated with the amide linkage. In addition, experimental values fromgas electron diffraction (“GED”) are provided for corroboration of thetheoretical results for DMF and DMA (Schultz et al., J. Phys. Chem.97:4966 (1993); Mack et al., J. Am. Chem. Soc. 119:3567 (1997), whichare hereby incorporated by reference in their entirety). The overallvalues show good agreement with the structural characteristics describedpreviously for SWNTs (Wildoer et al., Nature 391:59 (1998), which ishereby incorporated by reference in its entirety). The calculatedcarbon-carbon bond length in an SWNT is 1.42 Å, while the bond angle is120°. These results are suggestive of a potential π orbital stackingbetween the solvent and SWNTs, which is expected to be sensitive tothese geometries. The alkyl amide solvent which exhibits the mostfavorable structural alignment to the carbon-carbon bonding in an SWNTis DMA. Therefore, it is proposed that the highest SWNT dispersion limitobserved in DMA is a result of the highly polar π system whichstructurally contains an optimal geometry for interaction with the SWNTbackbone.

TABLE 2 MOPAC AM1 Theoretical Calculations for the Bond Length and BondAngles of Alkyl Amide Solvents* C═O C—N Bond Length Bond Length R—C═ON—C═O Solvent (Å) (Å) Bond Angle Bond Angle DMF 1.242 1.380 122.678°122.909° (1.224) (1.391) (119.5°) (123.5°) DMA 1.249 1.388 120.260°119.979° (1.226) (1.368) (123.1°) (121.0°) DEA 1.248 1.392 120.245°120.419° DMP 1.247 1.391 121.762° 120.164° *Experimental values from gaselectron diffraction (GED) studies are listed in parentheses withappropriate references.

Following the same protocol which was used with raw soot, purified SWNTswere dispersed in DMA, and the corresponding optical absorption spectrawere obtained. Selection of DMA was based upon it having the highestobserved dispersion limit for the raw SWNT soot. Shown in FIG. 5 a is anoverlay of the spectra used for Beer's Law analysis to calculate theestimated dispersion limit and extinction coefficients. The purifiedSWNT dispersion limit was determined to be 3.13 μg/mL in DMA, on thebasis of the described method. The presence of carboxylic acid groupsfrom the nitric acid processing (Chen et al., J. Phys. Chem. B 105:2525(2001), which is hereby incorporated by reference in its entirety) onthe purified SWNTs may influence the dispersion in DMA. However,purified-annealed SWNTs showed similar dispersion limits. The extinctioncoefficients for purified SWNTs in DMA were calculated to be 43.4 and39.0 mL·mg⁻¹·cm⁻¹ at 1.27 and 1.77 eV, respectively. Summarized in Table3 are the dispersion limit and extinction coefficient results comparingthe purified SWNTs with raw SWNT soot and nanostructured carbon. Thecharacteristic differences in absorption properties for each of thesecarbon-based materials can be observed in the spectral overlay shown inFIG. 5 b. The relationship between absorption intensity and peakresolution for the SWNT transitions can be directly related to the SWNTmass fraction of a sample. The calculated extinction coefficients can beused to estimate the relative SWNT concentration in a laser-generatedsample for the established diameter distribution andsemiconducting-to-metallic ratio. It is important to recognize that themagnitude of the extinction coefficient is dependent on both the solventeffects (Table 1) and SWNT sample purity (Table 3). Other potential SWNTphysical properties that may influence the extinction coefficientsinclude diameter, length, and bundling effects. These considerations mayaccount for the variation in extinction coefficients reported thus farfor SWNTs (Bahr et al., Chem. Commun. 2:193 (2001); Zhou et al., J.Phys. Chem. B 107:13588 (2003), which are hereby incorporated byreference in their entirety).

TABLE 3 Experimental Results for Dispersion Limit (D_(L)) Estimationsand Extinction Coefficients of Raw, Purified, and Nanostructred CarbonMaterials Dispersed in DMA ε_(1.27 eV) (mL · mg⁻¹ · ε_(1.77 eV) (mL ·mg⁻¹ · Solute D_(L) (μg/mL) cm⁻¹) cm⁻¹) raw 6.25 23.8 31.2 purified 3.1343.4 39.0 nano- 325 15.7 25.2 structured carbon

An important advantage to SWNT-solvent dispersions over other dispersingstrategies is the ability to easily remove the solvent throughevaporation and recover the starting material. The optical absorptionoverlay shown in FIG. 6 depicts the differences in purified SWNTs airsprayed from acetone onto a quartz slide (dry solid) and purified SWNTsdispersed in DMA. The enhanced resolution of the SWNT transitions forthe DMA dispersion may be attributed to a debundling effect, in analogywith optical absorption spectra of surfactant stabilized SWNTs(O'Connell et al., Science 297:593 (2002), which is hereby incorporatedby reference in its entirety). This implies that smaller bundles orindividual SWNTs are present in the DMA dispersion (O'Connell et al.,Science 297:593 (2002), which is hereby incorporated by reference in itsentirety). Additionally, SEM images were obtained from SWNTs that wereforced to aggregate into bundles from a DMA dispersion by solventevaporation, prior to analysis. The results for purified SWNTs beforeand after dispersion in DMA indicate the presence of bundles for bothcases, albeit the precipitated SWNTs appear to have a more straightened,“relaxed” structure (FIG. 7). Therefore, the dispersion process appearsto suspend the SWNTs without apparent damage to the underlyingstructure. The dispersion results described here are not unique topurified laser-generated SWNTs; similar dispersion behavior in DMA usingpurified HiPco (Chiang et al., J. Phys. Chem. B 105:8297 (2001), whichis hereby incorporated by reference in its entirety) has also beendemonstrated.

The capability for a series of alkyl amide solvents to disperseas-produced raw and purified SWNTs has been evaluated. Characterizationusing optical absorption spectroscopy has enabled calculation of thedispersion limit and extinction coefficients for the laser generatedsemiconducting and metallic SWNTs. The highly polar π system and optimalgeometries of the alkyl amide solvents are proposed to be the factorsresponsible for the dispersion of SWNTs. The best dispersion of thesolvents studied was in DMA, which also corresponded to the bestcombination of steric and electronic factors. These SWNT-DMA dispersionsproved to be stable for days to even weeks. The optical absorptionspectra generated from these dispersions showed well-resolved finestructure, presumably due to debundling. Production of stable SWNTdispersions without the use of any external agent (i.e. surfactant,polymer, amine, etc.) emerges as a powerful strategy towards probing theproperties of nanostructured carbon, such as SWNTs. Specifically, theextinction coefficients for purified nanostructured carbon can be usedto monitor the relative SWNT mass fraction during the purificationprocess. Ultimately, the use of nanostructured carbon-solventdispersions in electrophoretic separations may lead to phase pure SWNTson the basis of diameter and type.

Example 4 Nanostructured Carbon-Diamide Dispersions

In addition to the alkyl amide solvents described above, diamidecompounds have been verified to efficiently disperse carbon nanotubes.N,N,N′N′-tetramethylmalonamide (TMMA) has been shown to dispersepurified SWNTs at a concentration >25 μg/mL. The optical absorption datafor purified SWNTs in TMMA over a series of concentrations is set forthin FIG. 8. The corresponding values at the peak maxima for eachconcentration were plotted in FIG. 9 to establish the dispersion limitresults as described above. The results show that TMMA is capable ofdispersing SWNTs at a concentration >25 μg/mL since the data points donot deviate from the linear trendline indicative of a true Beer's lawrelationship.

Example 5 Electrophoretic Separation of SWNTs in Organic Solvents

A 1 μg/mL SWNT-DMA dispersion was placed in a glass U-tube (10 cmpathlength) containing 15 mL of pure DMA. Two copper electrodes wereinserted at the ends of the U-tube and a 30 V bias was placed betweenthem, resulting in a field of 300 V/m. The electrode where thedispersion was pipetted was the negative electrode and the positiveelectrode was where the extracts were removed. Initial attemptsevaluated the positive copper electrode to ensure that the SWNTs werebeing electrophoretically promoted through the DMA solution andattracted to the positive electrode. The result as seen in the SEM imageof FIGS. 10 a-b is that the SWNTs were depositing on the copperelectrode within 30 minutes.

The next series of experiments involved removing volumetric extracts (˜1mL) and determining the changes in the optical absorption spectra, whichis a direct measure of the concentration and electronic types(semiconducting and metallic) in solution. The data in FIG. 11illustrates the distinct spectral features of SWNTs with the peak at1.27 eV arising from the electronic transitions in semiconducting SWNTsand the one at 1.77 eV from metallic SWNTs. The results from the twoextracts after 30 and 60 minutes are that the absorbance peak ratio isdramatically different than the stock solution. Based on the ratio ofabsorbance values, the approximate degree of “enrichment” in metallictypes compared to the stock solution is from 40% in the stock to 60% inthe first extract. Extract 1 shows the largest enrichment due to thehigher mobility in metallic SWNTs which is due to their larger inherentdielectric constant. The second extract is still metallically-enriched,but the results suggest that after an hour, semiconducting species alsomove with sufficient mobility to be extracted at the positive electrode.The peaks from 2.1 eV and up represent both semiconducting and metallictransitions and are in qualitative agreement with the enrichmentobservations, but are not a good measure of the degree of enrichment.

Example 6 Purity Assessment of SWNTs, Using Optical Absorption—Synthesisand Purification

SWNTs were synthesized using the pulsed laser vaporization technique(L-SWNTs), employing an Alexandrite laser (755 nm). The laser pulse wasrastered (corner to corner over 1 cm² with 50% overlap of 100 μs pulsesat a repetition rate of 10 Hz) over the surface of a graphite (1-2micron) target doped with 2% w/w Ni (submicron) and 2% w/w Co (<2micron), at an average power density of 100 W/Cm². The reaction furnacetemperature was maintained at 1150° C., with a chamber pressure of 700torr under 100 sccm flowing Ar in a 46 mm inner diameter (50 mm outerdiameter) quartz tube (Landi et al., J. Phys. Chem. B 108:17089 (2004),which is hereby incorporated by reference in its entirety). Synthesis ofa representative nanostructured carbon component in the raw L-SWNT sootwas performed by laser vaporization at the described conditions for anundoped graphite target. For comparison of material properties andpurity assessment protocols from commercial sources, a batch ofarc-discharge SWNTs (A-SWNTs) was purchased from Carbon Solutions, Inc(Riverside, Calif.). The corresponding non-SWNT carbon representative ofthe arc-discharge process was used in the form of carbon soot, made fromthe resistive heating of graphite rods, and purchased from Aldrich (St.Louis, Mo.).

Purification of both L- and A-SWNT raw soots was performed based on apreviously reported procedure (Landi et al., J. Phys. Chem. B 108:17089(2004), which is hereby incorporated by reference in its entirety). Insummary, 50-100 mg of each raw SWNT soot was brought to reflux at 120°C. in 3M nitric acid for 16 hours, and then filtered over a 1 μm PTFEmembrane filter with copious amounts of water. The membrane filters weredried at 70° C. in vacuo to release the resulting SWNT papers from thefilter paper. The L-SWNT paper was thermally oxidized in air at 450° C.for 1 hour in a Thermolyne 1300 furnace, followed by a 6M hydrochloricacid wash for 30 minutes using magnetic stirring, equivalent filteringsteps, and a final oxidation step at 550° C. for 1 hour. In the case ofthe A-SWNTs, a sequential series of thermal oxidations at 450, 525, and600° C. occurred for 1 hour each, with intervening 6M HCl acid wash andfiltering steps being performed. The typical purification yield for thelaser raw soot was 10% w/w and the arc raw soot was 2% w/w. However,concern for the quality of purified material was a larger considerationthan optimization of this purification process.

Example 7 Material Characterization

Characterization of the SWNTs, nanostructured carbon, and carbon sootwas performed by SEM, Raman spectroscopy, and TGA. The SEM was performedat 2 kV using a Hitachi S-900, with samples applied directly to thebrass stub using silver paint. Raman spectroscopy was performed at roomtemperature using a JY-Horiba Labram spectrophotometer from 100-2800cm⁻¹ with excitation energies of 1.96 eV (He/Ne) and 2.54 eV (Ar). Theseenergies have been shown to probe both the metallic and semiconductingSWNTs, respectively, over the range of diameters for L- and A-SWNTs usedin this study (Dresselhaus et al., Carbon 40:2043 (2002), which ishereby incorporated by reference in its entirety). TGA was conductedusing a TA Instruments 2950. Samples were placed in the platinum panbalance in quantities of ˜1 mg and ramped at 10° C./min from roomtemperature up to 950° C. under air at a gas flow rate of 60 sccm.

Example 8 Optical Absorption and Constructed Sample Sets

UV-vis-NIR spectra were obtained on stable dispersions of SWNTs,nanostructured carbon, and carbon soot in DMA and an air sprayed sampleon a quartz slide from a 0.1 mg/mL SWNT-acetone solution using a PerkinElmer Lambda 900 spectrophotometer (Landi et al., J. Phys. Chem. B108:17089 (2004), which is hereby incorporated by reference in itsentirety). Sample handling for dispersion solutions involved the use of1 cm quartz cuvettes. Data was obtained over an energy range of0.90-4.25 eV, corresponding to the transmission window of the alkylamide solvent. Given the fact that the spectrophotometer is onlymeasuring optical transmittance during these measurements, thepossibility exists that scattering in the dispersions will affect theabsorption data. Although the effects of particle scattering are quitepronounced on sprayed SWNT samples (>30%) (Lebedkin et al., J. Phys.Chem. 107:1949 (2003), which is hereby incorporated by reference in itsentirety), previous reports indicate that scattering on SWNT dispersionsis negligible at concentrations of 10 μg/mL in DMF (Itkis et al., NanoLett. 3:309 (2003), which is hereby incorporated by reference in itsentirety) and 20 μg/mL in aqueous surfactant dispersions (Lebedkin etal., J. Phys. Chem. 107:1949 (2003), which is hereby incorporated byreference in its entirety). Therefore, the dispersion concentrationemployed presently of 2.5 μg/mL should exhibit even less spectraldistortion due to scattering than those reported. The constructed lasersample set was prepared from a volumetric mixture of a 2.5 μg/mL stocksolution of purified L-SWNTs in DMA with a 2.5 μg/mL stock solution ofnanostructured carbon-DMA in 20% increments (i.e. concentrations of 0%,20%, 40%, 60%, 80%, and 100% w/w SWNTs). The same procedure was alsoemployed for preparation of the constructed arc sample set withvolumetric mixing of a 2.5 μg/mL purified A-SWNT-DMA dispersion with a2.5 μg/mL stock solution of CS-DMA. The raw L- and A-SWNT soots werealso analyzed from 2.5 μg/mL DMA dispersions. Each of theseconcentrations reflects the mass of carbon containing materialcalculated from TGA data, adjusting for the relative mass of metaloxides (assuming Ni/Co metal in laser raw soot is 72% and Ni/Y metal inarc raw soot is 75% of the residual oxide value for 50:50 metal catalystmixtures) as determined by the decomposition residue.

Example 9 Fullerene Extraction

Evaluation of the C₆₀ fullerene content in the carbonaceous samples wasperformed by a toluene extraction and analysis by optical absorptionspectroscopy based on the reported extinction coefficient (∈=54,200 M⁻¹cm⁻¹ at 336 nm for C₆₀ in toluene) (Bachilo et al., J. Phys. Chem.105:9845 (2001), which is hereby incorporated by reference in itsentirety). The carbonaceous samples were prepared at concentrations of˜0.25 mg/mL in toluene (an order of magnitude below the solubility limitof C₆₀ in toluene of 2.8 mg/mL (Dresselhaus et al., Science ofFullerenes and Carbon Nanotubes, Academic Press: San Diego, Calif.(1996), which is hereby incorporated by reference in its entirety)) andplaced in an ultrasonic bath for 20 minutes at 25° C. Aftercentrifugation for 10 minutes at 5,000 rpm, the supernatant was analyzedto determine the soluble fullerenes content in toluene.

Example 10 Purity Assessment Methods

The accuracy of purity assessment methods can be validated byappropriate characterization with a reference SWNT sample set. Theopportunity to acquire a 100% w/w pure SWNT sample is currently notavailable (Itkis et al., Nano Lett. 3:309 (2003), which is herebyincorporated by reference in its entirety), thereby necessitating analternative reference. A reference sample set has been constructed wherevarious SWNT mass fractions are achieved through the mixtures ofpurified SWNTs and representative carbonaceous by-products. Purifiedlaser (L-) and arc (A-) SWNTs were prepared by standard purificationtreatment and shown to possess a high degree of purity and materialquality based on conventional microscopy and spectroscopy. The absolutedegree of purity of the purified SWNTs is obviously unknown. However,verification of the assessment method is not dependent on the absolutequality of the purified SWNTs, but is based on the correlation of purityestimates with designed fractions in the constructed sample sets. Thepurity assessment protocols will remain valid even with other referencesamples of potentially higher purity (Itkis et al., Nano Lett. 3:309(2003), which is hereby incorporated by reference in its entirety).

Example 11 Material Characterization and Constructed Sample Sets

Initially, microscopic and spectroscopic characterization was performedon the raw SWNTs, purified SWNTs, and representative carbonaceoussynthesis impurities (i.e. nanostructured carbon for laser synthesis andcarbon soot for arc-discharge synthesis) to identify correspondingsample morphologies and physical properties. Shown in FIGS. 12 a-f arethe SEM images of (a) raw L-SWNTs, (b) raw A-SWNTs, (c) purifiedL-SWNTs, (d) purified A-SWNTs, (e) nanostructured carbon, and (f) carbonsoot. Evident from the raw L-SWNTs are the highly entangled bundles ofSWNTs with the obvious presence of amorphous carbon and metal catalystimpurities. Similar morphologies are seen with the raw A-SWNTs, althoughthe observed quantity of SWNTs is generally lower than in the case ofL-SWNTs. The purified SWNTs in both cases show highly ordered, welldefined bundles with trace metal particles present. Analysis of the SEMimages for nanostructured carbon and carbon soot show nearly identicalsurface morphology with globular particles and the absence of any carbonnanotube materials. The TGA data shows significantly higher metalcatalyst impurities in the raw A-SWNTs compared to the raw L-SWNTs(31.9% w/w vs. 10.2% w/w, respectively). The purified A- and L-SWNTsshow residual impurities of 16.9 and 11.7% w/w, respectively, andsignificantly higher carbonaceous decomposition temperatures compared tothe raw soots based on the 1^(st) derivative plots (i.e., 730° C. and767° C., respectively). The nanostructured carbon and carbon soot TGAdata show nearly complete carbonaceous decomposition by ˜625° C.However, the presence of graphitic material is apparent in the carbonsoot with a minor decomposition transition at ˜700° C.

Raman spectroscopy was performed on each of the material types using aHe/Ne 1.96 eV laser and the spectra are shown in FIG. 13. The pronouncedD- (˜1300 cm⁻¹) and G-Bands (˜1600 cm⁻¹) are evident in thenanostructured carbon and carbon soot samples with a higher relativeintensity around 500 cm⁻¹ for the carbon soot attributed to increasedfullerene content (Dresselhaus et al., Science of Fullerenes and CarbonNanotubes, Academic Press: San Diego, Calif. (1996), which is herebyincorporated by reference in its entirety). This observation wasconfirmed by the toluene extraction of soluble C₆₀ fullerenes wherenanostructured carbon and carbon soot contain 2.6% w/w and 3.6% w/w,respectively. The absence of a radial breathing mode (“RBM”) (˜120-200cm⁻¹) for both nanostructured carbon and carbon soot samples supportsthe notion that these samples lack any SWNTs. In comparison, the typicalspectra for SWNTs is observed in both purified arc and laser with theprominent RBM peaks, G- (˜1400-1600 cm⁻¹), and G′-Bands (˜2600 cm⁻¹)being distinctly observed (Dresselhaus et al., Carbon 40:2043 (2002),which is hereby incorporated by reference in its entirety). Although thepresence of functional groups may exist from an oxidizing acidpurification treatment (Hu et al., J. Phys. Chem. B 107:13838 (2003),which is hereby incorporated by reference in its entirety), theextremely weak D-Band (˜1320 cm⁻¹) in the Raman spectra for bothpurified L- and A-SWNTs is evidence for a high degree of crystallinityand relatively few defects related to functionalization in carbonnanotubes (Dresselhaus et al., Carbon 40:2043 (2002); Endo et al., Appl.Phys. Lett. 79:1531 (2001), which are hereby incorporated by referencein their entirety). Further analysis of the diameter distributions wasmade with a complementary Ar laser at 2.54 eV for the purified laser andarc-SWNTs. Using an expression for bundled SWNTs, the correspondingdiameter distributions were calculated to range from 1.2-1.5 nm forlaser and 1.3-1.6 nm for arc (Rao et al., Phys. Rev. Lett. 86:3895(2001), which is hereby incorporated by reference in its entirety).Thus, these samples exhibit a slight difference in the diameterdistribution while still containing both semiconducting and metallictypes (Kataura et al., Synth. Met. 103:2555 (1999), which is herebyincorporated by reference in its entirety). Overall, based on themicroscopic and spectroscopic data, the nanostructured carbon and carbonsoot show similar sample morphology and are appropriate representationsof carbon impurities (i.e. 0% SWNT samples) for laser and arc-dischargesynthesis strategies, respectively. Likewise, the purified L- andA-SWNTs represent a high degree of carbonaceous purity and are furtherdenoted as the “100%” SWNT samples for each synthesis' constructedsample set.

Constructed sample sets were prepared as previously described byvolumetric mixing of stock DMA dispersions of purified SWNTs (“100%”)with the respective carbon impurity constituent (nanostructured carbonfor laser and carbon soot for arc). However, it should be noted that thechoice of carbon impurities will affect the constructed sample sets.Selection of nanostructured carbon is a highly suitable material sinceit was manufactured in the same laser synthesis reactor. Since anequivalent carbon impurity is currently unavailable from the raw arcSWNT soot vendor, the selection of carbon soot as the most appropriatechoice for a representative material has been made because it wasproduced under similar conditions, albeit from a different vendor. Theconcentration calculations for the DMA dispersions included adjustmentfrom decomposition residue values from the TGA data such that theresulting 2.5 μg/mL is indicative of the carbonaceous content in thesolutions. FIGS. 14 a-b display the characteristic optical absorptiondata for the DMA dispersions over the solvent transmittance window of0.90 eV to 4.25 eV. Evident from the data are the well resolved peaksdue to the electronic transitions associated with the Van Hovesingularities in the density of states for SWNTs (Kataura et al., Synth.Met. 103:2555 (1999), which is hereby incorporated by reference in itsentirety). The two prominent peaks at ˜1.2 eV and ˜1.7 eV originate fromthe second interband transitions of semiconducting SWNTs (^(S)E₂₂) andfirst interband transitions of metallic SWNTs (^(M)E₁₁), respectively(Kataura et al., Synth. Met. 103:2555 (1999); Lian et al., J. Phys.Chem. B 107:12082 (2003), which are hereby incorporated by reference intheir entirety). The overall absorption in a SWNT-containing sample is asuperposition of absorbances due to the interband electronic transitionsand background π-plasmons of both the SWNTs and carbon impurities. Thedramatic change in absorption intensity as a function of ^(c)w_(SWNTs)for both sample sets provides the necessary reference standard forcomparison of purity assessment methods.

Example 12 Linear Subtraction Approach

Recent attempts at purity assessment have utilized a linear subtractionover selected spectral cutoffs to estimate the absorption from non-SWNTelectronic transitions for a SWNT-containing sample (Itkis et al., NanoLett. 3:309 (2003); Hu et al., J. Phys. Chem. B 107:13838 (2003); Itkiset al., J. Phys. Chem. B 108:12770 (2004); Sen et al., Chem. Mater.15:4273 (2003); Zhao et al., J. Phys. Chem. B 108:8136 (2004); Lian etal., J. Phys. Chem. B 108:8848 (2004), which are hereby incorporated byreference in their entirety). Application of the method to the currentpurified SWNTs shows a calculated areal reference ratio for purifiedlaser SWNTs of 0.319 and for purified arc SWNTs equal to 0.253 (Itkis etal., Nano Lett. 3:309 (2003), which is hereby incorporated by referencein its entirety). These values are dramatically larger than the reportedvalue of 0.141 for the high purity arc reference sample or the purifiedsample (EA-P2=0.186), further supporting the quality (Lian et al., J.Phys. Chem. B 108:8848 (2004), which is hereby incorporated by referencein its entirety) of the purified SWNTs used in this study. The selectionof the dispersion solvent (DMA instead of DMF) cannot be attributed asthe main factor in these differences since the extinction coefficientsfor SWNTs calculated for each solvent vary by less than 2%. Although theconstructed sample sets do support the qualitative notion that the peakintensity at ^(S)E₂₂ is directly related to the SWNT purity, propagationof a linear subtraction beyond the spectral cutoffs fails to accuratelyreflect the underlying absorption due to the SWNT π-plasmon and othercarbonaceous absorption features (see FIGS. 15 a-b). Application of theareal ratio of the linearly subtracted ^(S)E₂₂ peak to the total areaover selected spectral cutoffs (0.97 eV-1.53 eV for laser and 0.94eV-1.47 eV for arc) for the constructed sample sets, significantlydeviates from the designed fractions. Shown in FIG. 16 and representedby the closed data points (values are in Table 4) are the calculatedSWNT fractions for both laser and arc sample sets using the currentreference ratios (0.319 for purified laser and 0.253 for purified arc)as a function of the constructed SWNT mass fraction, ^(c)w_(SWNTs). Itis apparent from the experimental results for two different SWNTchirality distributions, that the reported method overestimates theactual SWNT fraction.

TABLE 4 Purity Assessment Results for the Constructed SWNT Sample SetsBased on the Nonlinear Lorentzian Estimation of the π-Plasmon for SWNTsin a 2.5 μg/mL DMA Dispersion As Compared to the Areal Absorbance Methodfrom the Literature and the Modified Linear Subtraction^(a) DesignedFraction Modified Linear Modified Linear ^(C)W_(SWNTs) Itkis et al.^(S)E₂₂ ^(M)E₁₁ π-Sub.^(S)E₂₂ π-Sub.^(M)E₁₁ 100% Laser Ref. = 0.319^(a)Ref. = 0.0143^(a) Ref. = 0.00615^(b) 101 (1) 103  (3) 80% Laser 86  (8)79 (1) 81 (1) 80 80% 86  (8) Laser 60% Laser 70 (17) 58 (3) 61 (2) 5960% 70 (17) Laser 40% Laser 53 (33) 39 (3) 43 (8) 39 40% 53 (33) Laser20% Laser 28 (40) 18 (10)  24 (20)  20 20% 28 (40) Laser Avg. % Dev. 9(24) 2 (4) 2 (8) 1 (1) 2  (3) (Avg. Rel. % Error) 100% Arc Ref. =0.253^(c) Ref. = 0.00974^(c) Ref. = 0.00370^(d) 104 (4) 106  (6) 80% Arc96 (20) 81 (1) 84 (5) 78 80% 96 (20) Arc 60% Arc 84 (40) 61 (2) 63 (5)59 60% 84 (40) Arc 40% Arc 61 (53) 38 (5) 50 (25)  35 40% 61 (53) Arc20% Arc 45 (125)  24 (20)  30 (50)  22 20% 45 (125)  Arc Avg. % Dev. 22(59) 2 (7) 7 (21)  3 (6) 4  (7) (Avg. Rel. % Error) Raw Laser 31 22 2824 30 Raw Arc 31 22 22 19 26 ^(a) Integration limits = 0.97-1.53 eV,^(b) Integration limits = 1.53-2.08 eV, ^(c) Integration limits =0.94-1.47 eV, ^(d) Integration limits = 1.47-2.00 eV

Using a modified linear peak ratio, it is possible to estimate therelative purity of SWNTs at equal solvent dispersion concentrations. Themodified linear ratio calculation involves dividing the area of the peakregion above the linear fit for an unknown SWNT sample[^((P,X))AA(^(S)E₂₂)] by the established reference value for a puresample [^((P,R))AA(^(S)E₂₂)]. Since the constructed samples are at equalconcentrations, 2.5 μg/mL, the modified linear ratio can be applied andthe results are represented by the open data points (values are in Table4) of FIG. 16. The reference values for the modified linear peak areasof the purified laser and purified arc SWNT samples are shown in Table 4as the “100%” samples. The average percent deviation is <7% in allcases, but the average percent error increases significantly with lowerSWNT designed fractions. Since selection of the spectral cutoffs forintegration is based on the SWNT reference sample, the limitationsassociated with any linear approach are problematic, including overlapof electronic transitions and peak minima shifting from absorption ofimpurities in the unknown SWNT samples. Such effects are proposed toaccount for the higher average percent errors in the calculation at lowSWNT concentrations. For a more accurate SWNT purity assessment,knowledge of the underlying contributions from the electronictransitions and π-plasmons of all constituents (i.e., SWNTs andcarbonaceous impurities) is warranted. In addition, the development of arapid assessment protocol which is concentration independent is alsodesirable, since the modified linear approach requires equal dispersionconcentrations for proper analysis.

Example 13 Nonlinear Subtraction Approach

In a given SWNT-containing sample, the total absorption at a selectedenergy will be a superposition of intensities from interband electronictransitions and the π-plasmon for each carbonaceous constituent. If thetotal π-plasmon contribution can be subtracted from the opticalabsorption spectrum, the resulting data will reflect only the interbandelectronic transitions of the SWNT materials. Therefore, appropriatemodeling of the π-plasmon background and the subsequent datasubtraction, will enable the development of calibration curves thatdirectly relate to the relative concentrations of each carbonaceouscomponent. Based on these calibration curves, it is expected that asimple relationship for measuring the SWNT purity can be obtained. Incomparison to a linear subtraction approach, the nonlinear functionwould account for physical properties in the SWNT absorption spectrumlike peak overlap and transition broadening, which cannot be adequatelyaccounted for by selection of integration limits at neighboring peakminima.

Although the underlying π-plasmon over the SWNT absorption range hasbeen previously approximated by a linear relationship (Itkis et al.,Nano Lett. 3:309 (2003); Hu et al., J. Phys. Chem. B 107:13838 (2003);Itkis et al., J. Phys. Chem. B 108:12770 (2004); Sen et al., Chem.Mater. 15:4273 (2003); Zhao et al., J. Phys. Chem. B 108:8136 (2004),which are hereby incorporated by reference in their entirety), varioustheoretical and experimental reports have investigated the nonlinearfunctional dependence of this transition (Stockli et al., Phys. Rev. B64:115424 (2001); Shyu et al., Phys. Rev. B 62:8508 (2000); Shyu et al.,Phys. Rev. B 60:14434 (1999); Lin et al., Phys. Rev. B 62:13153 (2000);Lin et al., Phys. Rev. B 53:15493 (1996); Lin et al., Phys. Rev. B50:17744 (1994); Jiang et al., Phys. Rev. B 54:13487 (1996); Lauret etal., Phys. Rev. Lett. 90:57404 (2003); Bose et al., Phys. Lett. A289:255 (2001); Pichler et al., Phys. Rev. Lett. 80:4729 (1998), whichare hereby incorporated by reference in their entirety). The π-plasmonis the collective excitations of the electrons associated with theπ-band in a sample (Pichler et al., Phys. Rev. Lett. 80:4729 (1998),which is hereby incorporated by reference in its entirety). The peakmaximum of the dispersion relation, which corresponds to the π-plasmonresonance energy (ω_(π) for h=1), are generally reported in the range of˜4.5-7 eV. The use of a Lorentzian function to model a wide variety ofphysical observables involving resonance behavior is well established(Dresselhaus et al., Science of Fullerenes and Carbon Nanotubes;Academic Press: San Diego, Calif., 1996); Lauret et al., Phys. Rev.Lett. 90:57404 (2003), which are hereby incorporated by reference intheir entirety). Furthermore, surface plasmons can be modeled by adriven damped harmonic oscillator, whose solution is indeed a Lorentzianin the underdamped regime (Jackson, Plane Electromagnetic Waves and WavePropagation. In “Classical Electrodynamics,” 2^(nd) ed.; John Wiley &Sons: New York, 1962, p 284, which is hereby incorporated by referencein its entirety). Therefore, a Lorentzian function has been selected tomodel the carbonaceous π-plasmon in the following form:

${L(x)} = \frac{a}{1 + \left( \frac{x - b}{c} \right)^{2}}$where a is the peak amplitude, b equals the peak energy, and c is ameasure of the peak width. Analyses were performed using theKaleidagraph software package.

Example 14 Lorentzian Curve Fitting Results

In the case of the constructed sample sets, the position of theπ-plasmon peak is outside the solvent transmission window for DMA.Therefore, validation of the Lorentzian functional form's efficacy infitting the π-plasmon peak, was initially made by applying the aboveequation to an acetone sprayed sample of purified SWNTs where the peakis observable. FIG. 17 shows the Lorentzian curve fit to the opticaltransmission data in the peak region of 4.0-5.0 eV, for the purifiedL-SWNT sample. The observed correlation between the curve fit and datasubstantiates the selection of a Lorentzian for modeling of theπ-plasmon. However, it is inappropriate to use the calculated curvefitting parameters from this fit, including the peak value of ˜4.5 eV,in other non-acetone sprayed samples, especially for the SWNT-DMAdispersions. This assertion is based on major differences inexperimental acquisition conditions. The effects of SWNT bundling,solvent interactions during dispersion, or thin film particle scatteringin the sprayed sample, are several examples of these differences.Instead, the Lorentzian curve fits for the constructed sample sets weregenerated from the optical absorption data between 4.00-4.25 eV, wherethe π-plasmon absorption is proposed to be dominant in comparison to theelectronic transitions of the carbonaceous materials. The resultingcurve fits are shown for the DMA dispersions containing purified L-SWNT,π-SWNT, nanostructured carbon, and carbon soot in FIGS. 18 and 19. TheLorentzian curve fits for the purified SWNTs are expected to representthe superposition of π-plasmon contributions from the entiredistribution of SWNT diameters and electronic types present in eachsample. Similarly, the Lorentzian curve fits for nanostructured carbonand carbon soot are a superposition of the π-plasmon contributions fromthe various carbonaceous components, including the overall morphologyand particle sizes in each sample. There is a strong correlation betweenthe optical absorption data and chosen Lorentzian functional form.Sensitivity of this approach to the range of data for curve fitting wasalso investigated, but the most reasonable agreement with the spectralfeatures of the SWNTs was observed for the prescribed data range. Slightvariations in the fitting region (e.g. 3.75-4.00 eV and 3.75-4.25 eV)resulted in little or no change in the low energy range (˜1-2 eV) wherethe purity assessment is performed. The value of the Lorentzian curvefits at 1.5 eV showed a deviation of less than 5% for the data rangesexamined. Therefore, the Lorentzian function was deemed an appropriateapproximation to the π-plasmon for the constructed sample sets.

The Lorentzian curve fits determined the π-plasmon resonance energy tobe at 4.84 eV for the purified L-SWNTs and 4.90 eV for the purifiedA-SWNTs. As expected, the location of the π-plasmon peak positions haveshifted between sprayed solid sample and solvent dispersion due to thedifferences in experimental conditions. However, the measured andprojected values are consistent with experimental reports using opticalabsorption spectroscopy of evaporated SWNTs where the peak was stated tobe “around 5 eV.”

The curve fits for the π-plasmons of nanostructured carbon (ω_(π)=5.26eV) and carbon soot (ω_(π)=5.29 eV) show a general trend whereby theresonance energy is higher than for the purified SWNTs in the DMAdispersions. This would be consistent with a lowering of the energyrequired for the collective π-π* transitions in SWNTs since thehighly-conjugated π-system leads to delocalization of the electrons. Theabsorption of C₆₀ fullerenes which occurs at 3.7 eV is below theemployed curve fit range, but may influence the curve fitting approachif significant quantities are present. Since in both nanostructuredcarbon and carbon soot the presence of C₆₀ was low (˜3% w/w), consistentwith the lack of any spectral signature, the contribution to the curvefit is considered negligible.

In the constructed sample sets, the Lorentzian curve fits are asuperposition of all the π-plasmon contributions present in the sample,including both SWNT and carbonaceous impurity distributions. Theresulting nonlinear π-plasmon functions showed similar curve fittingparameters for both laser and arc constructed sample sets. The resultingoptical absorption spectra, after subtraction of the nonlinear π-plasmonfunctions, are shown in FIGS. 20 a-b for both constructed sample sets.It is important to note that any effects due to particle scattering inthe DMA dispersions, which have previously been reported as negligiblefor DMF and aqueous dispersions at higher concentrations, may be used asa further refinement to the ascribed nonlinear subtraction. The dataindicates a broad absorption over the energy range for bothnanostructured carbon and carbon soot materials which are attributed tothe electronic transitions of the constituent carbonaceous materials(such as fullerenes, amorphous carbon, and other graphitic components).The purified L-SWNTs and A-SWNTs display well resolved peak structures,particularly in the region above 2 eV, representative of the electronictransitions for each diameter and electronic type. Such spectra areexpected to enable the appropriate deconvolution of absorption databased the predicted energy gap transitions for individual SWNTdiameters. This type of analysis will allow for the calculation of thesemiconducting:metallic ratio (S:M) for any SWNT diameter distributionand serve as a means for accurate assessment of electronic typeseparations (Samsonidze et al., Appl. Phys. Lett. 85:1006 (2004), whichis hereby incorporated by reference in its entirety).

An observed result from the nonlinear curve fits is that the minimabetween peaks (^(S)E₂₂ and ^(M)E₁₁ for example) do not equal zero. Thisresult is consistent with the expectation that an optical absorptionspectrum of SWNTs exhibits an overlap of electronic transitions and peakbroadening for individual transitions. Such physical properties are notaccounted for in a linear subtraction model, and are important factorsin determining an appropriate π-plasmon subtraction. Overlap ofelectronic transitions may exist due to variation in the density ofstates for the collection of diameters and chiralities involved with theoptical absorption. Therefore, with the typical diameter distributionranging from 1.2-1.5 nm for laser (1.3-1.6 nm for arc), the possibilityexists that an optical absorption peak from a smaller diametersemiconducting SWNT could overlap with a larger diameter metallic SWNT.In addition, peak broadening in the case of SWNTs could be influenced bystructural distortions and defects, finite SWNT lengths, or bundlingeffects arising from the van der Waals interactions between SWNTs.

Example 15 Calibration Curves for Purity Assessment

Each of the designed fractions in FIGS. 20 a-b coincides with theappropriate superposition of absorbance expected from the constructeddilutions between nanostructured carbon and L-SWNTs (A-SWNTs).Therefore, it is possible to establish calibration curves for both laserand arc SWNTs utilizing the constructed sample sets. FIGS. 21 a-bdemonstrate the linear relationships that result from selection of theabsorbance values at the peak maxima for the semiconducting (A^(S)E₂₂)and metallic (A^(M)E₁₁) transitions in both constructed sample sets. Theanticipated correlation between the data and the linear curve fit isclearly observed. The calibration lines are unique to the SWNTconstructed sample sets from FIGS. 20 a-b and therefore cannot be usedas an absolute metric for material comparison with any SWNT sample ordiameter distribution. Development of such calibration curves is alsorelative to the initial choice of carbon impurity used to fabricate theconstructed sample sets (nanostructured carbon for laser and carbon sootfor arc). The results, however, clearly demonstrate the methodology forSWNT purity assessment of typical laser and arc-synthesized materials.

There are important considerations related to the choice of analysisused to generate the calibration curves (i.e., peak maxima versusintegrated peak area). In the present case, the use of peak maximavalues has important advantages over integrated area based on theobserved SWNT distribution. For the laser and arc ^(S)E₂₂ and ^(M)E₁₁peaks in FIGS. 20 a-b, the superposition of interband electronictransitions based on the samples' diameter ranges give rise to a peakeddistribution with the amplitude proportional to the area with minimaleffects of peak overlap (as described above) and appropriate selectionof integration limits. The degree of overlap is largely influenced bythe type of SWNT distribution being evaluated, since the density ofinterband electronic transitions over a particular energy range dictatesthis overlap as visualized from a Kataura plot. In samples where theinterband peaks result in multi-modal structures and selection ofintegration limits is less problematic, the integrated area approach mayprove worthwhile.

Application of the calculated linear trendlines in FIGS. 21 a-b isspecific to 2.5 μg/mL DMA dispersion, but this approach is valid for anyconcentration below the dispersion limit. The purity assessment resultsas seen in Table 4 are dramatically improved compared to the reportedareal absorbance ratio method and consistently show better correlationwith the lower designed SWNT mass fractions than the modified linearsubtraction. The calculated fractions using the nonlinear π-plasmonsubtraction are within an average deviation of the designed values by1-2% for the L-SWNTs and 3-4% for the A-SWNTs. In comparison, themodified linear subtraction has slightly higher deviation of 2-7%depending on the SWNT sample and selected peak. Another consideration isthe accuracy of the calculated value to the designed value, representedby the average relative percent error shown in parentheses in Table 4.For the nonlinear π-plasmon subtracted approach, the L-SWNTs exhibit anaverage relative error within 3% of any designed fraction and theA-SWNTs are within 7% for either peak. The modified linear subtractionshows relatively good agreement for the ^(S)E₂₂ peaks in the laser andarc sample sets, but shows significantly higher average relative errorsfrom the ^(M)E₁₁ peak.

Modifications in the SWNT diameter distributions and (S:M) may alter theempirical calibration curves, but the nonlinear curve fitting approachhas demonstrated success for both laser and arc SWNTs. The naturalextension of these results from the constructed sample sets is toevaluate raw laser and arc SWNT soots with the established nonlinearπ-plasmon fit and calibration curves. Shown in FIGS. 22 a-b are theoptical absorption spectra for 2.5 μg/mL DMA dispersions of raw L- andA-SWNT soots, including the corresponding Lorentzian curve fits for theraw soots. The π-plasmon subtracted data are depicted in the insets forboth L-SWNT and A-SWNT soots with the corresponding “0%” and “100%” SWNTsubtracted spectra also shown as a reference for comparison. In theacquired data, there is a slight increase in the absorption of the rawlaser soot as compared to the purified spectra between ˜2-3 eV evidentby the offset in the overlay. This increase in absorption is proposed toresult from the carbonaceous coatings that are produced during synthesisand are removed during the high temperature oxidation treatments in thepurification. It is expected that the carbonaceous coatings will have aneffect on the observed π-plasmon energy, and indeed there aredifferences between the raw SWNT soots and the expected values from theconstructed sample sets. The offset is more dramatic in the case of theraw arc soot, thereby suggesting that the degree of carbonaceouscoatings would be higher. However, the raw arc soot may have originallycontained graphitic material. The possibility then exists that thepurified A-SWNTs has retained this graphitic constituent since thepurification process may not be efficient at its removal. The effects ofC₆₀ and other fullerenes can also impact the optical absorption data ofraw SWNT soot, but current results from a toluene extraction of the rawlaser and raw arc materials show only trace quantities (<1% w/w).Therefore, the higher absorption in this range is likely a combinationof effects in the raw soots, but does not significantly impact purityassessment through analysis of the ^(S)E₂₂ peak. The data indicates thatthe ^(S)E₂₂ peak is most accurate due to confounding effects ofcarbonaceous coatings from synthesis. The results using the calibrationcurves for the ^(S)E₂₂ peak in FIGS. 21 a-b on the π-plasmon subtractedraw SWNT soot data are shown in Table 4. The raw L-SWNT soot and A-SWNTsoot were calculated to equal 24% and 19% w/w SWNTs of the totalcarbonaceous material, respectively. As expected, these values aresubstantially lower than the calculated 31% for both laser and arc basedon the areal absorbance ratio method using the current reference ratios(0.319 for purified laser and 0.253 for purified arc). However, themodified linear subtraction is relatively consistent with the π-plasmonsubtracted results indicating a value of 22% w/w for each raw SWNT soot.When the TGA data (raw L-SWNT soot residue=10.2% and raw A-SWNT sootresidue=31.9% w/w) is accounted for, the resulting total mass fractionfrom the π-plasmon subtraction results is 22% w/w for raw laser and 13%w/w for raw arc. These values are strikingly lower than previousreports, but are consistent with the microscopic analysis in FIG. 4.

Example 16 Rapid Purity Assessment Protocols

While the nonlinear π-plasmon model has been demonstrated as aphysically robust approach, the generation and implementation ofcalibration curves are not time efficient for large numbers of samplesand do not allow for critical evaluation of results in presentations orpublications without access to the actual data. Therefore, it is alsodesirable to establish rapid assessment protocols which facilitate SWNTsample screening during synthesis, purity monitoring during purificationprocedures, and enable the estimation of purity from visual observationduring presentations or in publications. Based on the constructed samplesets, there are several important observations which can be incorporatedinto purity assessment protocols. As shown in FIG. 23, the absoluteintensity and ratio of intensities of the ^(S)E₂₂ peak and ^(M)E₁₁ peakfor the L-SWNT constructed sample set (as well as for the A-SWNTconstructed sample set) varies consistently with the relative weightfraction of SWNTs present. Further visualization can be made by drawingtie lines from peak maxima and observing the changes in slope of theline as a function of weight fraction. Therefore, such observationspresent alternative strategies for SWNT purity assessment.

Both laser and arc constructed sample sets were evaluated based on asummation of the absorbance values for the ^(S)E₂₂ and ^(M)E₁₁ peaks asa function of the designed SWNT fraction. The results are shown in FIG.24 a and illustrate the linear relationship for both cases which arenormalized to dispersion concentration (i.e., absorbance values weredivided by the dispersion concentration of 2.5 μg/mL). The offset iny-intercepts are attributed to the differences in the maximum absorptionwhich can result from different extinction coefficients (SWNT orselected carbon impurities), SWNT molecular weights, or purity betweenpurified L- and A-SWNTs. (Such an offset in the absorption for purifiedA-SWNTs can be related to any residual graphite in the sample, therebyaltering the DMA dispersion concentration during analysis.) Evaluationof the slope for peak maxima tie lines also results in a lineartrendline as depicted in FIG. 24 b. The data are also normalized todispersion concentration and show a convergence of maximum negativeslope (approaching −0.012) for the “100%” sample in both laser and arcconstructed sample sets. For peak maxima tie lines of zero slope, thepurity is approximately 70% w/w SWNTs in the carbonaceous component.This result implies that positive tie line slopes will occur for samplesof less than 70% w/w SWNTs and negative slopes exist for samples ofgreater purity. Comparison of the peak maxima summation or tie lineslope for a sample with unknown SWNT purity to the empirical relations(normalized to the dispersion concentration of 1 μg/mL) in FIGS. 24 a-bwill allow for the rapid determination of its carbonaceous purity.

The ^(S)E₂₂ peak was previously shown to be least affected by theabsorption from the carbon impurities in a sample (FIGS. 20 a-b), andfrom the carbonaceous coatings (FIGS. 22 a-b). Therefore, the use ofthis peak for a rapid assessment method is plausible and simple. Shownin FIG. 25 a are the data for the absorbance value of the ^(S)E₂₂ peakfor both L-SWNT and A-SWNT constructed sample sets normalized todispersion concentration. The resulting linearity represents a simpleand straightforward method to rapidly assess SWNT purity. A lineardependence is also observed when the ratios of peak maximum absorbancevalues were plotted as shown in FIG. 25 b. Both constructed sets ofpurified SWNT samples exhibit a peak maxima ratio approaching 1.2 in theplot. A similar observation to the slope approach is that for a ratio(A^(S)E₂₂/A^(M)E₁₁) of 1.0 (corresponding to a zero tie line slope), thepurity of a given sample is approximately 70% w/w SWNTs in thecarbonaceous component. Therefore, the same notion exists where peakmaxima ratios greater than 1.0 will occur for samples greater than 70%w/w SWNTs and ratios lower than 1.0 exist for purity levels less than70% w/w SWNTs. In comparison to the other rapid assessment methods, thepeak maxima ratio is extremely attractive since it is independent of thedispersion concentration (i.e., the concentration terms cancel upondivision), provided the dispersion being analyzed is below theSWNT-solvent dispersion limit. The ratio of peak absorbances can resultin an important derivation whereby convolution of the purity with the(S:M) and ratio of extinction coefficients also occurs:

$\frac{{A\left( {}^{S}E_{22} \right)} \equiv {ɛ_{S} \star l \star c_{S}}}{{A\left( {}^{M}E_{11} \right)} \equiv {ɛ_{M} \star l \star c_{M}}} = {\left( \frac{ɛ_{S}}{ɛ_{M}} \right) \star \left( \frac{\zeta}{1 - \zeta} \right)}$where c_(S)=ζ*c_(T) and c_(M)=(1−ζ)*c_(T). This equation represents theratio of peak maxima for a pure SWNT sample given the fact that there isno concentration term for the impurities. The variables ∈_(S) and ∈_(M)are the extinction coefficients for semiconducting and metallic SWNTs,respectively, l is the optical pathlength, c_(S) is the concentration ofsemiconducting, c_(M) is the concentration of metallic SWNTs, c_(T) isthe total SWNT concentration, and ζ is the fraction of the total SWNTconcentration that is semiconducting. As a result, the derivation showsthat (S:M) is equal to ζ/(1−ζ).

If the (S:M) is assumed to remain constant (typically reported at 2:1),then appropriate use of extinction coefficients in a Beer's Law analysisyields another method for purity assessment. While overlap of SWNTelectronic transitions can influence an experimentally calculated ∈, thevalue derived from selecting the peak maximum will be the leastsensitive to this effect. As described in the examples above, previouswork has determined the value of the extinction coefficients fornanostructured carbon, raw, and purified L-SWNT-DMA dispersions. Asimilar analysis has been made using carbon soot and purified A-SWNTsfor comparison with the laser data. Shown in Table 5 are the calculatedextinction coefficients derived from a Beer's Law analysis involvingserial dilution of the 2.5 μg/mL DMA dispersions for each material withpure DMA. These results are consistent with the previous values. Sincethe mass used to calculate the dispersion concentrations for the Beer'sLaw analysis was for the total concentration of purified SWNTs, c_(T),the magnitude of the extinction coefficient values are a convolution ofthe extinction properties for the inherent (S:M). Therefore, as long asthe SWNT samples being analyzed have a similar (S:M), incorporation ofthese constants into the equation below will result in a relativedetermination of the carbonaceous concentration ratio between the totalbulk SWNTs (c_(T)) and carbonaceous impurities (c_(CI)):

$\frac{c_{T}}{c_{CI}} = \frac{\left( {{A_{M_{E_{11}}} \cdot ɛ_{{CI}_{S_{E_{22}}}}} - {A_{S_{E_{22}}} \cdot ɛ_{{CI}_{M_{E_{11}}}}}} \right)}{\left( {{A_{S_{E_{22}}} \cdot ɛ_{M_{E_{11}}}} - {A_{{M_{E}}_{11}} \cdot ɛ_{S_{E_{22}}}}} \right)}$where$c_{W_{SWNTs}} = {\frac{\left( \frac{c_{T}}{c_{CI}} \right)}{\left( {1 + \frac{c_{T}}{c_{CI}}} \right)}.}$

TABLE 5 Calculated Extinction Coefficients of SWNTs, NanostructuredCarbon, and Carbon Soot in DMA Dispersions at Selected Energies forBeer's Law Analysis ε (^(S)E₂₂) ε (^(M)E₁₁) solute (mL · mg⁻¹ · cm⁻¹)(mL · mg⁻¹ · cm⁻¹) Carbon Soot (Aldrich) 10.5 (1.18 eV) 16.5 (1.65 eV)Nanostructured Carbon 15.5 (1.21 eV) 26.8 (1.78 eV) Purified A-SWNTs35.0 (1.18 eV) 30.0 (1.65 eV) Purified L-SWNTs 45.5 (1.21 eV) 40.0 (1.78eV)

Extinction coefficients for SWNTs have been reported in the literature,but an uncertainty exists in regard to what these values represent.Disparities in the value of the dispersion limit for the various speciespresent in a SWNT-containing sample (i.e., SWNTs, graphitic carbon,amorphous carbon, and/or fullerenes) call into question what material isthe major contributing factor to the measured extinction coefficients.For unambiguous interpretation of any extinction coefficient data, it isimperative that measurements be performed below the dispersion limit ofpurified SWNTs in the solvent (e.g. ˜3 μg/mL for a DMA dispersion).

The results from each of the rapid techniques on the constructed samplesets are listed in Table 6. Also included are the rapid assessmentresults on the 2.5 μg/mL raw laser and arc SWNT-DMA dispersions. Theproposed effects of carbonaceous coatings are most apparent in the peaksummation approach for both raw samples. However, the results for rawlaser SWNTs are consistent across each of the techniques with only aminor depression in values compared to the nonlinear π-plasmon model. Incontrast, the raw arc SWNT soot shows significantly more variability ineach of the rapid assessment analyses, specifically with the slopeapproach where the measured value is outside the calculated calibrationrange. As suggested earlier, this variability is attributed to thepresence of increased carbonaceous coatings or graphitic components inthe arc sample that artificially inflate the absorption in the ^(M)E₁₁peak range, thereby altering the purity assessment results. While thepeak summation and tie line slope approach are useful techniques, theyare also dispersion concentration dependent. In comparison, the peakmaxima ratio (A^(S)E₂₂/A^(M)E₁₁) and Beer's Law strategies areconcentration independent, provided the SWNT-organic solvent dispersionis below the dispersion limit (˜3 μg/mL for purified SWNTs). With theseobservations in mind, the recommended rapid approach to purityassessment includes the following simple procedure: (1) perform TGAanalysis of SWNT sample; (2) use corrected carbonaceous mass from TGAresidue to prepare and analyze a 2.5 μg/mL SWNT-DMA dispersion from0.90-4.25 eV using optical absorption spectroscopy; (3) evaluate theabsorbance peak maximum values for the ^(S)E₂₂ and ^(M)E₁₁ peaks; (4)calculate the peak maximum ratio and compare to the calibration curve orutilize Beer's Law with appropriate extinction coefficients for anassessment of the ^(c)w_(SWNTs) in the carbonaceous component; (5) andadjust for the metal catalyst impurities from the TGA residue todetermine the overall w_(SWNTs) in the analyzed sample.

TABLE 6 Purity Assessment Results for the Constructed SWNT Sample SetsBased on the Rapid Protocols^(a) Designed Fraction A(^(S)E₂₂) + Slope^(C)W_(SWNTs) A(^(M)E₁₁) [(A(^(S)E₂₂), A(^(M)E₁₁)] A(^(S)E₂₂)A(^(S)E₂₂)/A(^(M)E₁₁) Beer's Law 100% Laser 99 (1) 100 (0) 100 (0) 96(4) 99 (1) 80% Laser 80 (0) 80 (0) 80 (0) 80 (0) 79 (1) 60% Laser 61 (2)60 (0) 60 (0) 62 (3) 58 (3) 40% Laser 40 (0) 39 (3) 40 (0) 40 (0) 38 (5)20% Laser 20 (0) 20 (0) 20 (0) 17 (15)  20 (0) Avg. % Dev. <1 (1) <1 (1)0 (0) 2 (5) 1 (2) (Avg. Rel % Error) 100% Arc 103 (3) 101 (1) 102 (2) 97(3) 99 (1) 80% Arc 78 (3) 80 (0) 79 (1) 82 (3) 80 (0) 60% Arc 56 (7) 59(2) 58 (3) 64 (7) 60 (0) 40% Arc 39 (3) 34 (15)  39 (3) 37 (8) 37 (8)20% Arc 21 (5) 21 (5) 22 (10)  19 (5) 24 (20)  Avg. % Dev. 2 (4) 2 (5) 2(4) 3 (5) 2 (6) (Avg. Rel. % Error) Raw Laser 34 17 28 19 20 Raw Arc 53— 42 9 18 ^(a)The values in parentheses represent the average relativepercent error for a measured fraction, CWSWNTs.

Although preferred embodiments have been depicted and described indetail herein, it will be apparent to those skilled in the relevant artthat various modifications, additions, substitutions, and the like canbe made without departing from the spirit of the invention and these aretherefore considered to be within the scope of the invention as definedin the claims which follow.

1. A dispersion of nanostructured carbon in an organic solvent, whereinthe organic solvent comprises an alkyl amide compound having thestructure:

wherein R₁ and R₂ are independently selected from the group consistingof H, C₁-C₆ alkyl, and phenyl, provided that R₁ and R₂ are not both H,and R₃ is selected from the group consisting of C₁-C₆ alkyl, phenyl, and

wherein Z is N or CH; R₄, R₅, R₆, and R₇ are independently selected fromthe group consisting of H, C₁-C₆ alkyl, and phenyl; and n is an integerfrom 1 to
 3. 2. The dispersion according to claim 1, wherein R₁, R₂, andR₃ are independently selected from C₁-C₆ alkyl and phenyl.
 3. Thedispersion according to claim 2, wherein the alkyl amide compound isselected from the group consisting of N,N-dimethylacetamide,N,N-dimethylpropanamide, and N,N-diethylacetamide.
 4. The dispersionaccording to claim 3, wherein the alkyl amide compound isN,N-dimethylacetamide.
 5. The dispersion according to claim 1, whereinR₃ is

R₁, R₂, R₄, and R₅ are independently selected from C₁-C₆ alkyl andphenyl.
 6. The dispersion according to claim 5, wherein Z is N and nis
 1. 7. The dispersion according to claim 6, wherein the alkyl amidecompound is selected from the group consisting ofN,N,N′,N′-tetramethylmalonamide, N,N′-dibutyl-N,N′-dimethylmalonamide,N,N,N′,N′-tetra(isopropyl)malonamide, N,N,N′,N′-tetrahexylmalonamide,2-methyl-N,N,N′,N′-tetrahexylmalonamide, andN,N,N′,N′-tetrahexyl-2,2-dimethylmalonamide.
 8. The dispersion accordingto claim 7, wherein the alkyl amide compound isN,N,N′,N′-tetramethylmalonamide.
 9. The dispersion according to claim 5,wherein Z is CH, n is 1, and R₄ and R₅ are H.
 10. The dispersionaccording to claim 9, wherein the alkyl amide compound isN,N-dimethylacetoacetamide.
 11. The dispersion according to claim 5,wherein Z is N and n is
 2. 12. The dispersion according to claim 11,wherein the alkyl amide compound is N,N,N′,N′-tetramethylsuccinamide.13. The dispersion according to claim 1, wherein the nanostructuredcarbon is selected from the group consisting of raw nanostructuredcarbon soot, purified nanostructured carbon, and mixtures thereof. 14.The dispersion according to claim 1, wherein the nanostructured carbonis selected from the group consisting of carbon nanotubes, nano-onions,nano-horms, and fullerenes.
 15. The dispersion according to claim 14,wherein the nanostructured carbon comprises carbon nanotubes selectedfrom the group consisting of single wall carbon nanotubes, double wallcarbon nanotubes, and multi-wall carbon nanotubes.
 16. The dispersionaccording to claim 15, wherein the nanostructured carbon comprisessingle wall carbon nanotubes.
 17. A method of mobilizing nanostructuredcarbon, said method comprising: providing a dispersion of nanostructuredcarbon according to claim 1 and applying an electrical field to thedispersion under conditions effective to mobilize the nanostructuredcarbon.
 18. The method according to claim 17, wherein the nanostructuredcarbon comprises single wall carbon nanotubes.
 19. The method accordingto claim 17 further comprising: depositing the nanostructured carbononto a substrate.
 20. The method according to claim 19, wherein thesubstrate is selected from the group consisting of a metal electrode anda doped semiconductor.
 21. A method of dispersing nanostructured carbonin an organic solvent, said method comprising: providing nanostructuredcarbon and contacting the nanostructured carbon with an organic solventcomprising an alkyl amide compound having the structure:

wherein R₁ and R₂, are independently selected from the group consistingof H, C₁-C₆ alkyl, and phenyl, provided that R₁ and R₂ are not both H,and R₃ is selected from the group consisting of H, C₁-C₆ alkyl, phenyland

wherein Z is N or CH; R₄, R₅, R₆, and R₇ are independently selected fromthe group consisting of H, C₁ to C₆ alkyl, and phenyl; and n is aninteger from 1 to 3 under conditions effective to disperse thenanostructured carbon in the organic solvent.
 22. The method accordingto claim 21, wherein R₁, R₂, and R₃ are independently selected fromC₁-C₆ alkyl and phenyl.
 23. The method according to claim 22, whereinthe alkyl amide compound is selected from the group consisting ofN,N-dimethylacetamide, N,N-dimethylpropanamide, andN,N-diethylacetamide.
 24. The method according to claim 23, wherein thealkyl amide compound is N,N-dimethylacetamide.
 25. The method accordingto claim 21, wherein R₃ is

R₁, R₂, R₄, and R₅ are independently selected from C₁-C₆ alkyl andphenyl.
 26. The method according to claim 25, wherein Z is N and n is 1.27. The method according to claim 26, wherein the alkyl amide compoundis selected from the group consisting ofN,N,N′,N′-tetramethylmalonamide, N,N′-dibutyl-N,N′-dimethylmalonamide,N,N,N′,N′-tetra(isopropyl)malonamide, N,N,N′,N′-tetrahexylmalonamide,2-methyl-N,N,N′,N′-tetrahexylmalonamide, andN,N,N′,N′-tetrahexyl-2,2-dimethylmalonamide.
 28. The method according toclaim 27, wherein the alkyl amide compound isN,N,N′,N′-tetramethylmalonamide.
 29. The method according to claim 25,wherein Z is CH, n is 1, and R₄ and R₅ are H.
 30. The method accordingto claim 29, wherein the alkyl amide compound isN,N-dimethylacetoacetamide.
 31. The method according to claim 25,wherein n is
 2. 32. The method according to claim 31, wherein the alkylamide compound is N,N,N′,N′-tetramethylsuccinamide.
 33. The methodaccording to claim 21, wherein the nanostructured carbon is selectedfrom the group consisting of raw nanostructured carbon soot, purifiednanostructured carbon, and mixtures thereof.
 34. The method according toclaim 21, wherein the nanostructured carbon is selected from the groupconsisting of carbon nanotubes, nano-onions, nano-horms, and fullerenes.35. The method according to claim 34, wherein the nanostructured carboncomprises carbon nanotubes selected from the group consisting of singlewall carbon nanotubes, double wall carbon nanotubes, and multi-wallcarbon nanotubes.
 36. The method according to claim 35, wherein thenanostructured carbon comprises single wall carbon nanotubes.